[BUGFIX beta] Ensures that observers are flushed after CPs are updated

Ensures that observers are only flushed after CPs have fully updated.
Currently, they will fire before the `lastRevision` cache has been
updated, so they won't fire with the correct values or at the correct
times.

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

@@ -29,7 +29,7 @@ import {
 import expandProperties from './expand_properties';
 import { setObserverSuspended } from './observer';
 import { defineProperty } from './properties';
-import { notifyPropertyChange } from './property_events';
+import { beginPropertyChanges, endPropertyChanges, notifyPropertyChange } from './property_events';
 import { set } from './property_set';
 import { tagForProperty, update } from './tags';
 import { consume, track } from './tracked';
@@ -607,19 +607,24 @@ export class ComputedProperty extends ComputedDescriptor {
     }
 
     if (EMBER_METAL_TRACKED_PROPERTIES) {
-      let ret = this._set(obj, keyName, value);
+      let ret;
 
-      finishLazyChains(obj, keyName, ret);
+      try {
+        beginPropertyChanges();
+        ret = this._set(obj, keyName, value);
 
-      let propertyTag = tagForProperty(obj, keyName);
+        finishLazyChains(obj, keyName, ret);
 
-      if (this._dependentKeys !== undefined) {
-        update(propertyTag, getChainTagsForKeys(obj, this._dependentKeys));
-      }
+        let propertyTag = tagForProperty(obj, keyName);
 
-      setLastRevisionFor(obj, keyName, propertyTag.value());
+        if (this._dependentKeys !== undefined) {
+          update(propertyTag, getChainTagsForKeys(obj, this._dependentKeys));
+        }
 
-      return ret;
+        setLastRevisionFor(obj, keyName, propertyTag.value());
+      } finally {
+        endPropertyChanges();
+      }
     } else {
       return this.setWithSuspend(obj, keyName, value);
     }

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

@@ -101,6 +101,37 @@ moduleFor(
       assert.equal(count, 1, 'should have invoked observer');
     }
 
+    // https://github.com/emberjs/ember.js/issues/18246
+    async ['@test observer should fire when computed property is modified'](assert) {
+      let obj = { bar: 'bar' };
+      defineProperty(
+        obj,
+        'foo',
+        computed('bar', {
+          get() {
+            return get(this, 'bar');
+          },
+          set(key, value) {
+            return value;
+          }
+        })
+      );
+
+      get(obj, 'foo');
+
+      let count = 0;
+      addObserver(obj, 'foo', function() {
+        assert.equal(get(obj, 'foo'), 'baz', 'should have invoked after prop change');
+        count++;
+      });
+
+      set(obj, 'foo', 'baz');
+      await runLoopSettled();
+
+      assert.equal(count, 1, 'should have invoked observer');
+      assert.equal(get(obj, 'foo'), 'baz', 'computed should have correct value');
+    }
+
     async ['@test observer should continue to fire after dependent properties are accessed'](
       assert
     ) {