fix(core): allow EmbeddedViewRef context to be updated

Currently `EmbeddedViewRef.context` is read-only which means that the only way to update
it is to mutate the object which can lead to some undesirable outcomes if the template
and the context are provided by an external consumer (see angular#24515).

These changes make the property writeable since there doesn't appear to be a specific
reason why it was readonly to begin with.

goldens/public-api/core/core.d.ts

@@ -299,7 +299,7 @@ export declare class ElementRef<T = any> {
 }
 
 export declare abstract class EmbeddedViewRef<C> extends ViewRef {
-    abstract get context(): C;
+    abstract context: C;
     abstract get rootNodes(): any[];
 }
 

packages/core/src/linker/view_ref.ts

@@ -93,7 +93,7 @@ export abstract class EmbeddedViewRef<C> extends ViewRef {
   /**
    * The context for this view, inherited from the anchor element.
    */
-  abstract get context(): C;
+  abstract context: C;
 
   /**
    * The root nodes for this embedded view.

packages/core/src/render3/view_ref.ts

@@ -58,6 +58,10 @@ export class ViewRef<T> implements viewEngine_EmbeddedViewRef<T>, viewEngine_Int
     return this._lView[CONTEXT] as T;
   }
 
+  set context(value: T) {
+    this._lView[CONTEXT] = value;
+  }
+
   get destroyed(): boolean {
     return (this._lView[FLAGS] & LViewFlags.Destroyed) === LViewFlags.Destroyed;
   }

packages/core/src/view/refs.ts

@@ -264,6 +264,10 @@ export class ViewRef_ implements EmbeddedViewRef<any>, InternalViewRef {
     return this._view.context;
   }
 
+  set context(value: any) {
+    this._view.context = value;
+  }
+
   get destroyed(): boolean {
     return (this._view.state & ViewState.Destroyed) !== 0;
   }

packages/core/test/acceptance/template_ref_spec.ts

@@ -273,4 +273,50 @@ describe('TemplateRef', () => {
          });
     });
   });
+
+  describe('context', () => {
+    @Component({
+      template: `
+      <ng-template #templateRef let-name="name">{{name}}</ng-template>
+      <ng-container #containerRef></ng-container>
+    `
+    })
+    class App {
+      @ViewChild('templateRef') templateRef!: TemplateRef<any>;
+      @ViewChild('containerRef', {read: ViewContainerRef}) containerRef!: ViewContainerRef;
+    }
+
+    it('should update if the context of a view ref is mutated', () => {
+      TestBed.configureTestingModule({declarations: [App]});
+      const fixture = TestBed.createComponent(App);
+      fixture.detectChanges();
+      const context = {name: 'Frodo'};
+      const viewRef = fixture.componentInstance.templateRef.createEmbeddedView(context);
+      fixture.componentInstance.containerRef.insert(viewRef);
+      fixture.detectChanges();
+
+      expect(fixture.nativeElement.textContent).toBe('Frodo');
+
+      context.name = 'Bilbo';
+      fixture.detectChanges();
+
+      expect(fixture.nativeElement.textContent).toBe('Bilbo');
+    });
+
+    it('should update if the context of a view ref is replaced', () => {
+      TestBed.configureTestingModule({declarations: [App]});
+      const fixture = TestBed.createComponent(App);
+      fixture.detectChanges();
+      const viewRef = fixture.componentInstance.templateRef.createEmbeddedView({name: 'Frodo'});
+      fixture.componentInstance.containerRef.insert(viewRef);
+      fixture.detectChanges();
+
+      expect(fixture.nativeElement.textContent).toBe('Frodo');
+
+      viewRef.context = {name: 'Bilbo'};
+      fixture.detectChanges();
+
+      expect(fixture.nativeElement.textContent).toBe('Bilbo');
+    });
+  });
 });