[BUGFIX beta] Expose mechanism to detect if a property is a computed.

This adds a private/intimate method for use by the ember-inspector
(and possibly other tools) to aid the migration to native decorator
version of `computed` (since `computed(function() {}) instanceof
ComputedProperty` is no longer usable).

From the docs:

```md
Allows checking if a given property on an object is a computed property.
For the most part, this doesn't matter (you would normally just access
the property directly and use its value), but for some tooling specific
scenarios (e.g. the ember-inspector) it is important to differentiate if
a property is a computed property or a "normal" property.
```

packages/@ember/-internals/metal/index.ts

@@ -1,4 +1,9 @@
-export { default as computed, _globalsComputed, ComputedProperty } from './lib/computed';
+export {
+  default as computed,
+  isComputed,
+  _globalsComputed,
+  ComputedProperty,
+} from './lib/computed';
 export { getCacheFor, getCachedValueFor, peekCacheFor } from './lib/computed_cache';
 export { default as alias } from './lib/alias';
 export { deprecateProperty } from './lib/deprecate_property';

packages/@ember/-internals/metal/lib/computed.ts

@@ -22,7 +22,11 @@ import {
   makeComputedDecorator,
   removeDependentKeys,
 } from './decorator';
-import { descriptorForDecorator, isClassicDecorator } from './descriptor_map';
+import {
+  descriptorForDecorator,
+  descriptorForProperty,
+  isClassicDecorator,
+} from './descriptor_map';
 import expandProperties from './expand_properties';
 import { defineProperty } from './properties';
 import { notifyPropertyChange } from './property_events';
@@ -842,6 +846,23 @@ export function computed(
   ) as ComputedDecorator;
 }
 
+/**
+  Allows checking if a given property on an object is a computed property. For the most part,
+  this doesn't matter (you would normally just access the property directly and use its value),
+  but for some tooling specific scenarios (e.g. the ember-inspector) it is important to
+  differentiate if a property is a computed property or a "normal" property.
+
+  This will work on either a class's prototype or an instance itself.
+
+  @static
+  @method isComputed
+  @for @ember/debug
+  @private
+ */
+export function isComputed(obj: object, key: string): boolean {
+  return Boolean(descriptorForProperty(obj, key));
+}
+
 export const _globalsComputed = computed.bind(null);
 
 export default computed;

packages/@ember/-internals/metal/lib/descriptor_map.ts

@@ -9,17 +9,17 @@ const DECORATOR_DESCRIPTOR_MAP: WeakMap<
 /**
   Returns the CP descriptor assocaited with `obj` and `keyName`, if any.
 
-  @method descriptorFor
+  @method descriptorForProperty
   @param {Object} obj the object to check
   @param {String} keyName the key to check
   @return {Descriptor}
   @private
 */
 export function descriptorForProperty(obj: object, keyName: string, _meta?: Meta | null) {
-  assert('Cannot call `descriptorFor` on null', obj !== null);
-  assert('Cannot call `descriptorFor` on undefined', obj !== undefined);
+  assert('Cannot call `descriptorForProperty` on null', obj !== null);
+  assert('Cannot call `descriptorForProperty` on undefined', obj !== undefined);
   assert(
-    `Cannot call \`descriptorFor\` on ${typeof obj}`,
+    `Cannot call \`descriptorForProperty\` on ${typeof obj}`,
     typeof obj === 'object' || typeof obj === 'function'
   );
 

packages/@ember/-internals/metal/tests/computed_test.js

@@ -4,6 +4,7 @@ import {
   getCachedValueFor,
   defineProperty,
   isClassicDecorator,
+  isComputed,
   get,
   set,
   isWatching,
@@ -17,6 +18,24 @@ let obj, count;
 moduleFor(
   'computed',
   class extends AbstractTestCase {
+    ['@test isComputed is true for computed property on a factory'](assert) {
+      let Obj = EmberObject.extend({
+        foo: computed(function() {}),
+      });
+
+      Obj.proto(); // ensure the prototype is "collapsed" / merged
+
+      assert.ok(isComputed(Obj.prototype, 'foo'));
+    }
+
+    ['@test isComputed is true for computed property on an instance'](assert) {
+      let obj = EmberObject.extend({
+        foo: computed(function() {}),
+      }).create();
+
+      assert.ok(isComputed(obj, 'foo'));
+    }
+
     ['@test computed property should be an instance of descriptor'](assert) {
       assert.ok(isClassicDecorator(computed(function() {})));
     }

packages/ember/index.js

@@ -227,6 +227,7 @@ Ember.Error = EmberError;
 Ember.Debug = {
   registerDeprecationHandler: EmberDebug.registerDeprecationHandler,
   registerWarnHandler: EmberDebug.registerWarnHandler,
+  isComputed: metal.isComputed,
 };
 
 // ****@ember/instrumentation****