diff --git a/packages/-ember-data/tests/integration/adapter/rest-adapter-test.js b/packages/-ember-data/tests/integration/adapter/rest-adapter-test.js
index 33eb086f920..48d264fabe6 100644
--- a/packages/-ember-data/tests/integration/adapter/rest-adapter-test.js
+++ b/packages/-ember-data/tests/integration/adapter/rest-adapter-test.js
@@ -2561,7 +2561,7 @@ module('integration/adapter/rest_adapter - REST Adapter', function(hooks) {
   });
 
   if (hasJQuery) {
-    test('on error appends errorThrown for sanity', function(assert) {
+    test('on error appends errorThrown for sanity', async function(assert) {
       assert.expect(2);
 
       let jqXHR = {
@@ -2581,12 +2581,12 @@ module('integration/adapter/rest_adapter - REST Adapter', function(hooks) {
         assert.ok(false);
       };
 
-      return run(() => {
-        return store.findRecord('post', '1').catch(err => {
-          assert.equal(err, errorThrown);
-          assert.ok(err, 'promise rejected');
-        });
-      });
+      try {
+        await store.findRecord('post', '1');
+      } catch (err) {
+        assert.equal(err, errorThrown);
+        assert.ok(err, 'promise rejected');
+      }
     });
 
     test('on error wraps the error string in an DS.AdapterError object', function(assert) {
diff --git a/packages/-ember-data/tests/unit/adapters/json-api-adapter/ajax-options-test.js b/packages/-ember-data/tests/unit/adapters/json-api-adapter/ajax-options-test.js
index f6df0e2381c..a6b18ff4a26 100644
--- a/packages/-ember-data/tests/unit/adapters/json-api-adapter/ajax-options-test.js
+++ b/packages/-ember-data/tests/unit/adapters/json-api-adapter/ajax-options-test.js
@@ -9,7 +9,12 @@ module('unit/adapters/json-api-adapter/ajax-options - building requests', functi
   setupTest(hooks);
 
   hooks.beforeEach(function() {
-    this.owner.register('adapter:application', JSONAPIAdapter.extend({ useFetch: true }));
+    this.owner.register(
+      'adapter:application',
+      class extends JSONAPIAdapter {
+        useFetch = true;
+      }
+    );
     this.owner.register('serializer:application', JSONAPISerializer.extend());
   });
 
diff --git a/packages/-ember-data/tests/unit/adapters/rest-adapter/ajax-options-test.js b/packages/-ember-data/tests/unit/adapters/rest-adapter/ajax-options-test.js
index a6bc87dabbd..9cf44f74c71 100644
--- a/packages/-ember-data/tests/unit/adapters/rest-adapter/ajax-options-test.js
+++ b/packages/-ember-data/tests/unit/adapters/rest-adapter/ajax-options-test.js
@@ -20,7 +20,12 @@ module('unit/adapters/rest-adapter/ajax-options - building requests', function(h
     this.owner.register('model:person', Person);
     this.owner.register('model:place', Place);
 
-    this.owner.register('adapter:application', RESTAdapter.extend({ useFetch: true }));
+    this.owner.register(
+      'adapter:application',
+      class extends RESTAdapter {
+        useFetch = true;
+      }
+    );
     this.owner.register('serializer:application', RESTSerializer.extend());
   });
 
@@ -243,7 +248,12 @@ module('unit/adapters/rest-adapter/ajax-options - building requests', function(h
 
   module('ajax-options - ajax', function(hooks) {
     hooks.beforeEach(function() {
-      this.owner.register('adapter:application', RESTAdapter.extend({ useFetch: false }));
+      this.owner.register(
+        'adapter:application',
+        class extends RESTAdapter {
+          useFetch = false;
+        }
+      );
     });
 
     test('ajaxOptions() Content-Type is not set with ajax GET', function(assert) {
@@ -268,8 +278,14 @@ module('unit/adapters/rest-adapter/ajax-options - building requests', function(h
       assert.notOk(ajaxOptions.contentType, 'contentType not set with POST no data');
     });
 
-    test('ajaxOptions() Content-Type is set with ajax POST with data', function(assert) {
+    test('ajaxOptions() Content-Type is set with ajax POST with data if not useFetch', function(assert) {
       let store = this.owner.lookup('service:store');
+      this.owner.register(
+        'adapter:application',
+        class extends RESTAdapter {
+          useFetch = false;
+        }
+      );
       let adapter = store.adapterFor('application');
 
       let url = 'example.com';
@@ -278,5 +294,26 @@ module('unit/adapters/rest-adapter/ajax-options - building requests', function(h
 
       assert.equal(ajaxOptions.contentType, 'application/json; charset=utf-8', 'contentType is set with POST');
     });
+
+    test('ajaxOptions() Content-Type is set with ajax POST with data if useFetch', function(assert) {
+      let store = this.owner.lookup('service:store');
+      this.owner.register(
+        'adapter:application',
+        class extends RESTAdapter {
+          useFetch = true;
+        }
+      );
+      let adapter = store.adapterFor('application');
+
+      let url = 'example.com';
+      let type = 'POST';
+      let ajaxOptions = adapter.ajaxOptions(url, type, { data: { type: 'post' } });
+
+      assert.equal(
+        ajaxOptions.headers['content-type'],
+        'application/json; charset=utf-8',
+        'contentType is set with POST'
+      );
+    });
   });
 });
diff --git a/packages/-ember-data/tests/unit/adapters/rest-adapter/group-records-for-find-many-test.js b/packages/-ember-data/tests/unit/adapters/rest-adapter/group-records-for-find-many-test.js
index 0ca40a3cb14..668a6e2c298 100644
--- a/packages/-ember-data/tests/unit/adapters/rest-adapter/group-records-for-find-many-test.js
+++ b/packages/-ember-data/tests/unit/adapters/rest-adapter/group-records-for-find-many-test.js
@@ -23,12 +23,12 @@ module('unit/adapters/rest_adapter/group_records_for_find_many_test - DS.RESTAda
     requests = [];
     lengths = [];
 
-    const ApplicationAdapter = RESTAdapter.extend({
-      coalesceFindRequests: true,
+    class ApplicationAdapter extends RESTAdapter {
+      coalesceFindRequests = true;
 
       findRecord(store, type, id, snapshot) {
         return { id };
-      },
+      }
 
       ajax(url, type, options) {
         requests.push({
@@ -48,8 +48,8 @@ module('unit/adapters/rest_adapter/group_records_for_find_many_test - DS.RESTAda
 
         let testRecords = options.data.ids.map(id => ({ id }));
         return EmberPromise.resolve({ testRecords: testRecords });
-      },
-    });
+      }
+    }
 
     this.owner.register('adapter:application', ApplicationAdapter);
     this.owner.register('serializer:application', RESTSerializer.extend());
diff --git a/packages/adapter/addon/index.js b/packages/adapter/addon/index.js
index 504bf6bddfc..8ac492e4779 100644
--- a/packages/adapter/addon/index.js
+++ b/packages/adapter/addon/index.js
@@ -55,7 +55,7 @@ import EmberObject from '@ember/object';
   @class Adapter
   @extends EmberObject
 */
-export default EmberObject.extend({
+export default class Adapter extends EmberObject {
   /**
     If you would like your adapter to use a custom serializer you can
     set the `defaultSerializer` property to be the name of the custom
@@ -77,7 +77,7 @@ export default EmberObject.extend({
     @property defaultSerializer
     @type {String}
   */
-  defaultSerializer: '-default',
+  defaultSerializer = '-default';
 
   /**
     The `findRecord()` method is invoked when the store is asked for a record that
@@ -113,7 +113,6 @@ export default EmberObject.extend({
     @param {Snapshot} snapshot
     @return {Promise} promise
   */
-  findRecord: null,
 
   /**
     The `findAll()` method is used to retrieve all records for a given type.
@@ -145,7 +144,6 @@ export default EmberObject.extend({
     @param {SnapshotRecordArray} snapshotRecordArray
     @return {Promise} promise
   */
-  findAll: null,
 
   /**
     This method is called when you call `query` on the store.
@@ -178,7 +176,6 @@ export default EmberObject.extend({
     @param {Object} adapterOptions
     @return {Promise} promise
   */
-  query: null,
 
   /**
     The `queryRecord()` method is invoked when the store is asked for a single
@@ -217,7 +214,6 @@ export default EmberObject.extend({
     @param {Object} adapterOptions
     @return {Promise} promise
   */
-  queryRecord: null,
 
   /**
     If the globally unique IDs for your records should be generated on the client,
@@ -251,7 +247,6 @@ export default EmberObject.extend({
       newly created record.
     @return {(String|Number)} id
   */
-  generateIdForRecord: null,
 
   /**
     Proxies to the serializer's `serialize` method.
@@ -278,7 +273,7 @@ export default EmberObject.extend({
   */
   serialize(snapshot, options) {
     return snapshot.serialize(options);
-  },
+  }
 
   /**
     Implement this method in a subclass to handle the creation of
@@ -321,7 +316,6 @@ export default EmberObject.extend({
     @param {Snapshot} snapshot
     @return {Promise} promise
   */
-  createRecord: null,
 
   /**
     Implement this method in a subclass to handle the updating of
@@ -373,7 +367,6 @@ export default EmberObject.extend({
     @param {Snapshot} snapshot
     @return {Promise} promise
   */
-  updateRecord: null,
 
   /**
     Implement this method in a subclass to handle the deletion of
@@ -417,7 +410,6 @@ export default EmberObject.extend({
     @param {Snapshot} snapshot
     @return {Promise} promise
   */
-  deleteRecord: null,
 
   /**
     By default the store will try to coalesce all `fetchRecord` calls within the same runloop
@@ -428,7 +420,7 @@ export default EmberObject.extend({
     @property coalesceFindRequests
     @type {boolean}
   */
-  coalesceFindRequests: true,
+  coalesceFindRequests = true;
 
   /**
     The store will call `findMany` instead of multiple `findRecord`
@@ -467,7 +459,6 @@ export default EmberObject.extend({
     @param {Array} snapshots
     @return {Promise} promise
   */
-  findMany: null,
 
   /**
     Organize records into groups, each of which is to be passed to separate
@@ -486,7 +477,7 @@ export default EmberObject.extend({
   */
   groupRecordsForFindMany(store, snapshots) {
     return [snapshots];
-  },
+  }
 
   /**
     This method is used by the store to determine if the store should
@@ -536,7 +527,7 @@ export default EmberObject.extend({
   */
   shouldReloadRecord(store, snapshot) {
     return false;
-  },
+  }
 
   /**
     This method is used by the store to determine if the store should
@@ -591,7 +582,7 @@ export default EmberObject.extend({
   */
   shouldReloadAll(store, snapshotRecordArray) {
     return !snapshotRecordArray.length;
-  },
+  }
 
   /**
     This method is used by the store to determine if the store should
@@ -627,7 +618,7 @@ export default EmberObject.extend({
   */
   shouldBackgroundReloadRecord(store, snapshot) {
     return true;
-  },
+  }
 
   /**
     This method is used by the store to determine if the store should
@@ -663,7 +654,7 @@ export default EmberObject.extend({
   */
   shouldBackgroundReloadAll(store, snapshotRecordArray) {
     return true;
-  },
-});
+  }
+}
 
 export { BuildURLMixin } from './-private';
diff --git a/packages/adapter/addon/json-api.js b/packages/adapter/addon/json-api.js
index 4cb605611b1..425884831ad 100644
--- a/packages/adapter/addon/json-api.js
+++ b/packages/adapter/addon/json-api.js
@@ -143,10 +143,10 @@ import RESTAdapter from './rest';
   @constructor
   @extends RESTAdapter
 */
-const JSONAPIAdapter = RESTAdapter.extend({
-  defaultSerializer: '-json-api',
+class JSONAPIAdapter extends RESTAdapter {
+  defaultSerializer = '-json-api';
 
-  _defaultContentType: 'application/vnd.api+json',
+  _defaultContentType = 'application/vnd.api+json';
 
   /**
     @method ajaxOptions
@@ -157,12 +157,12 @@ const JSONAPIAdapter = RESTAdapter.extend({
     @return {Object}
   */
   ajaxOptions(url, type, options = {}) {
-    let hash = this._super(url, type, options);
+    let hash = super.ajaxOptions(url, type, options);
 
     hash.headers['Accept'] = hash.headers['Accept'] || 'application/vnd.api+json';
 
     return hash;
-  },
+  }
 
   /**
     By default the JSONAPIAdapter will send each find request coming from a `store.find`
@@ -219,17 +219,17 @@ const JSONAPIAdapter = RESTAdapter.extend({
     @property coalesceFindRequests
     @type {boolean}
   */
-  coalesceFindRequests: false,
+  coalesceFindRequests = false;
 
   findMany(store, type, ids, snapshots) {
     let url = this.buildURL(type.modelName, ids, snapshots, 'findMany');
     return this.ajax(url, 'GET', { data: { filter: { id: ids.join(',') } } });
-  },
+  }
 
   pathForType(modelName) {
     let dasherized = dasherize(modelName);
     return pluralize(dasherized);
-  },
+  }
 
   updateRecord(store, type, snapshot) {
     const data = serializeIntoHash(store, type, snapshot);
@@ -237,7 +237,7 @@ const JSONAPIAdapter = RESTAdapter.extend({
     let url = this.buildURL(type.modelName, snapshot.id, snapshot, 'updateRecord');
 
     return this.ajax(url, 'PATCH', { data: data });
-  },
-});
+  }
+}
 
 export default JSONAPIAdapter;
diff --git a/packages/adapter/addon/rest.js b/packages/adapter/addon/rest.js
index 5a6108a242b..7f5b7a6018f 100644
--- a/packages/adapter/addon/rest.js
+++ b/packages/adapter/addon/rest.js
@@ -293,31 +293,30 @@ const hasJQuery = typeof jQuery !== 'undefined';
   @extends Adapter
   @uses BuildURLMixin
 */
-let RESTAdapter = Adapter.extend(BuildURLMixin, {
-  defaultSerializer: '-rest',
+class RESTAdapter extends Adapter.extend(BuildURLMixin) {
+  defaultSerializer = '-rest';
 
-  _defaultContentType: 'application/json; charset=utf-8',
+  _defaultContentType = 'application/json; charset=utf-8';
 
-  fastboot: computed({
+  @computed
+  get fastboot() {
     // Avoid computed property override deprecation in fastboot as suggested by:
     // https://deprecations.emberjs.com/v3.x/#toc_computed-property-override
-    get() {
-      if (this._fastboot) {
-        return this._fastboot;
-      }
-      return (this._fastboot = getOwner(this).lookup('service:fastboot'));
-    },
-    set(key, value) {
-      return (this._fastboot = value);
-    },
-  }),
+    if (this._fastboot) {
+      return this._fastboot;
+    }
+    return (this._fastboot = getOwner(this).lookup('service:fastboot'));
+  }
+
+  set fastboot(value) {
+    return (this._fastboot = value);
+  }
 
   /**
     @property useFetch
     @type {Boolean}
     @public
   */
-  useFetch: true,
 
   /**
     By default, the RESTAdapter will send the query params sorted alphabetically to the
@@ -374,7 +373,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
       newQueryParams[sortedKeys[i]] = obj[sortedKeys[i]];
     }
     return newQueryParams;
-  },
+  }
 
   /**
     By default the RESTAdapter will send each find request coming from a `store.find`
@@ -423,7 +422,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     @property coalesceFindRequests
     @type {boolean}
   */
-  coalesceFindRequests: false,
+  coalesceFindRequests = false;
 
   /**
     Endpoint paths can be prefixed with a `namespace` by setting the namespace
@@ -507,7 +506,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     let query = this.buildQuery(snapshot);
 
     return this.ajax(url, 'GET', { data: query });
-  },
+  }
 
   /**
     Called by the store in order to fetch a JSON array for all
@@ -532,7 +531,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     }
 
     return this.ajax(url, 'GET', { data: query });
-  },
+  }
 
   /**
     Called by the store in order to fetch a JSON array for
@@ -559,7 +558,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     }
 
     return this.ajax(url, 'GET', { data: query });
-  },
+  }
 
   /**
     Called by the store in order to fetch a JSON object for
@@ -587,7 +586,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     }
 
     return this.ajax(url, 'GET', { data: query });
-  },
+  }
 
   /**
     Called by the store in order to fetch several records together if `coalesceFindRequests` is true
@@ -625,7 +624,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
   findMany(store, type, ids, snapshots) {
     let url = this.buildURL(type.modelName, ids, snapshots, 'findMany');
     return this.ajax(url, 'GET', { data: { ids: ids } });
-  },
+  }
 
   /**
     Called by the store in order to fetch a JSON array for
@@ -670,7 +669,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     url = this.urlPrefix(url, this.buildURL(type, id, snapshot, 'findHasMany'));
 
     return this.ajax(url, 'GET');
-  },
+  }
 
   /**
     Called by the store in order to fetch the JSON for the unloaded record in a
@@ -714,7 +713,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
 
     url = this.urlPrefix(url, this.buildURL(type, id, snapshot, 'findBelongsTo'));
     return this.ajax(url, 'GET');
-  },
+  }
 
   /**
     Called by the store when a newly created record is
@@ -738,7 +737,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     const data = serializeIntoHash(store, type, snapshot);
 
     return this.ajax(url, 'POST', { data });
-  },
+  }
 
   /**
     Called by the store when an existing record is saved
@@ -763,7 +762,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     let url = this.buildURL(type.modelName, id, snapshot, 'updateRecord');
 
     return this.ajax(url, 'PUT', { data });
-  },
+  }
 
   /**
     Called by the store when a record is deleted.
@@ -780,7 +779,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     let id = snapshot.id;
 
     return this.ajax(this.buildURL(type.modelName, id, snapshot, 'deleteRecord'), 'DELETE');
-  },
+  }
 
   _stripIDFromURL(store, snapshot) {
     let url = this.buildURL(snapshot.modelName, snapshot.id, snapshot);
@@ -801,10 +800,10 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     }
 
     return expandedURL.join('/');
-  },
+  }
 
   // http://stackoverflow.com/questions/417142/what-is-the-maximum-length-of-a-url-in-different-browsers
-  maxURLLength: 2048,
+  maxURLLength = 2048;
 
   /**
     Organize records into groups, each of which is to be passed to separate
@@ -872,7 +871,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     });
 
     return groupsArray;
-  },
+  }
 
   /**
     Takes an ajax response, and returns the json payload or an error.
@@ -929,7 +928,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     }
 
     return new AdapterError(errors, detailedMessage);
-  },
+  }
 
   /**
     Default `handleResponse` implementation uses this hook to decide if the
@@ -944,7 +943,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
   */
   isSuccess(status, headers, payload) {
     return (status >= 200 && status < 300) || status === 304;
-  },
+  }
 
   /**
     Default `handleResponse` implementation uses this hook to decide if the
@@ -959,7 +958,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
   */
   isInvalid(status, headers, payload) {
     return status === 422;
-  },
+  }
 
   /**
     Takes a URL, an HTTP method and a hash of data, and makes an
@@ -1023,7 +1022,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
 
       adapter._ajax(hash);
     }, 'DS: RESTAdapter#ajax ' + type + ' to ' + url);
-  },
+  }
 
   /**
     @method _ajaxRequest
@@ -1032,7 +1031,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
   */
   _ajaxRequest(options) {
     jQuery.ajax(options);
-  },
+  }
 
   _fetchRequest(options) {
     let fetchFunction = fetch();
@@ -1044,7 +1043,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
         'cannot find the `fetch` module or the `fetch` global. Did you mean to install the `ember-fetch` addon?'
       );
     }
-  },
+  }
 
   _ajax(options) {
     if (this.useFetch) {
@@ -1054,7 +1053,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     } else {
       this._ajaxRequest(options);
     }
-  },
+  }
 
   /**
     @method ajaxOptions
@@ -1102,7 +1101,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     options.url = this._ajaxURL(options.url);
 
     return options;
-  },
+  }
 
   _ajaxURL(url) {
     if (get(this, 'fastboot.isFastBoot')) {
@@ -1126,7 +1125,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     }
 
     return url;
-  },
+  }
 
   /**
     @method parseErrorResponse
@@ -1144,7 +1143,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     }
 
     return json;
-  },
+  }
 
   /**
     @method normalizeErrorResponse
@@ -1166,7 +1165,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
         },
       ];
     }
-  },
+  }
 
   /**
     Generates a detailed ("friendly") error message, with plenty
@@ -1180,7 +1179,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     @param  {Object} requestData
     @return {String} detailed error message
   */
-  generatedDetailedMessage: function(status, headers, payload, requestData) {
+  generatedDetailedMessage(status, headers, payload, requestData) {
     let shortenedPayload;
     let payloadContentType = headers['content-type'] || 'Empty Content-Type';
 
@@ -1198,7 +1197,7 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
       payloadDescription,
       shortenedPayload,
     ].join('\n');
-  },
+  }
 
   /**
     @method buildQuery
@@ -1219,8 +1218,8 @@ let RESTAdapter = Adapter.extend(BuildURLMixin, {
     }
 
     return query;
-  },
-});
+  }
+}
 
 function ajaxSuccess(adapter, payload, requestData, responseData) {
   let response;
@@ -1401,72 +1400,79 @@ function ajaxOptions(options, adapter) {
 }
 
 if (DEPRECATE_NAJAX) {
-  RESTAdapter = RESTAdapter.extend({
-    /**
-      @method _najaxRequest
-      @private
-      @param {Object} options jQuery ajax options to be used for the najax request
-    */
-    _najaxRequest(options) {
-      if (typeof najax !== 'undefined') {
-        najax(options);
-      } else {
-        throw new Error(
-          'najax does not seem to be defined in your app. Did you override it via `addOrOverrideSandboxGlobals` in the fastboot server?'
-        );
+  /**
+    @method _najaxRequest
+    @private
+    @param {Object} options jQuery ajax options to be used for the najax request
+  */
+  RESTAdapter.prototype._najaxRequest = function(options) {
+    if (typeof najax !== 'undefined') {
+      najax(options);
+    } else {
+      throw new Error(
+        'najax does not seem to be defined in your app. Did you override it via `addOrOverrideSandboxGlobals` in the fastboot server?'
+      );
+    }
+  };
+
+  Object.defineProperty(RESTAdapter.prototype, 'useFetch', {
+    get() {
+      if (typeof this[UseFetch] === 'boolean') {
+        return this[UseFetch];
       }
-    },
 
-    useFetch: computed({
-      get() {
-        if (this[UseFetch]) {
-          return this[UseFetch];
+      // Mixin validates all properties. Might not have it in the container yet
+      let ENV = getOwner(this) ? getOwner(this).resolveRegistration('config:environment') : {};
+      // TODO: https://github.com/emberjs/data/issues/6093
+      let jQueryIntegrationDisabled = ENV && ENV.EmberENV && ENV.EmberENV._JQUERY_INTEGRATION === false;
+
+      let shouldUseFetch;
+      if (jQueryIntegrationDisabled) {
+        shouldUseFetch = true;
+      } else if (typeof najax !== 'undefined') {
+        if (has('fetch')) {
+          deprecate(
+            'You have ember-fetch and jquery installed. To use ember-fetch instead of najax, set `useFetch = true` in your adapter.  In 4.0, ember-data will default to ember-fetch instead of najax when both ember-fetch and jquery are installed in FastBoot.',
+            false,
+            {
+              id: 'ember-data:najax-fallback',
+              until: '4.0',
+            }
+          );
+        } else {
+          deprecate(
+            'In 4.0, ember-data will default to ember-fetch instead of najax in FastBoot.  It is recommended that you install ember-fetch or similar fetch polyfill in FastBoot and set `useFetch = true` in your adapter.',
+            false,
+            {
+              id: 'ember-data:najax-fallback',
+              until: '4.0',
+            }
+          );
         }
 
-        let ENV = getOwner(this).resolveRegistration('config:environment');
-        // TODO: https://github.com/emberjs/data/issues/6093
-        let jQueryIntegrationDisabled = ENV && ENV.EmberENV && ENV.EmberENV._JQUERY_INTEGRATION === false;
-
-        let shouldUseFetch;
-        if (jQueryIntegrationDisabled) {
-          shouldUseFetch = true;
-        } else if (typeof najax !== 'undefined') {
-          if (has('fetch')) {
-            deprecate(
-              'You have ember-fetch and jquery installed. To use ember-fetch instead of najax, set `useFetch = true` in your adapter.  In 4.0, ember-data will default to ember-fetch instead of najax when both ember-fetch and jquery are installed in FastBoot.',
-              false,
-              {
-                id: 'ember-data:najax-fallback',
-                until: '4.0',
-              }
-            );
-          } else {
-            deprecate(
-              'In 4.0, ember-data will default to ember-fetch instead of najax in FastBoot.  It is recommended that you install ember-fetch or similar fetch polyfill in FastBoot and set `useFetch = true` in your adapter.',
-              false,
-              {
-                id: 'ember-data:najax-fallback',
-                until: '4.0',
-              }
-            );
-          }
+        shouldUseFetch = false;
+      } else if (hasJQuery) {
+        shouldUseFetch = false;
+      } else {
+        shouldUseFetch = true;
+      }
 
-          shouldUseFetch = false;
-        } else if (hasJQuery) {
-          shouldUseFetch = false;
-        } else {
-          shouldUseFetch = true;
-        }
+      addSymbol(this, UseFetch, shouldUseFetch);
 
-        addSymbol(this, UseFetch, shouldUseFetch);
+      return shouldUseFetch;
+    },
 
-        return shouldUseFetch;
-      },
-      set(key, value) {
-        addSymbol(this, UseFetch, value);
-        return value;
-      },
-    }),
+    set(value) {
+      addSymbol(this, UseFetch, value);
+      return value;
+    },
+  });
+} else {
+  Object.defineProperty(RESTAdapter.prototype, 'useFetch', {
+    value() {
+      return true;
+    },
+    configurable: true,
   });
 }