feat(ElementInjector): throw if multiple directives define the same host injectable

relates to #2015

modules/angular2/src/core/compiler/element_injector.ts

@@ -466,8 +466,14 @@ export class ProtoElementInjector {
 
   private static _createHostInjectorBindingData(bindings: List<ResolvedBinding>,
                                                 bd: List<BindingData>) {
+    var visitedIds: Map<number, boolean> = MapWrapper.create();
     ListWrapper.forEach(bindings, b => {
       ListWrapper.forEach(b.resolvedHostInjectables, b => {
+        if (MapWrapper.contains(visitedIds, b.key.id)) {
+          throw new BaseException(
+              `Multiple directives defined the same host injectable: "${stringify(b.key.token)}"`);
+        }
+        MapWrapper.set(visitedIds, b.key.id, true);
         ListWrapper.push(bd, new BindingData(ProtoElementInjector._createBinding(b), LIGHT_DOM));
       });
     });

modules/angular2/test/core/compiler/element_injector_spec.ts

@@ -106,7 +106,7 @@ class NeedsDirectiveFromParent {
 @Injectable()
 class NeedsDirectiveFromParentOrSelf {
   dependency: SimpleDirective;
-  constructor(@Parent({self:true}) dependency: SimpleDirective) { this.dependency = dependency; }
+  constructor(@Parent({self: true}) dependency: SimpleDirective) { this.dependency = dependency; }
 }
 
 @Injectable()
@@ -478,9 +478,22 @@ export function main() {
           expect(pei.getBindingAtIndex(i).key.token).toBe(i);
         }
       });
+
+      it('should throw whenever multiple directives declare the same host injectable', () => {
+        expect(() => {
+          createPei(null, 0, [
+            DirectiveBinding.createFromType(SimpleDirective, new dirAnn.Component({
+              hostInjector: [bind('injectable1').toValue('injectable1')]
+            })),
+            DirectiveBinding.createFromType(SomeOtherDirective, new dirAnn.Component({
+              hostInjector: [bind('injectable1').toValue('injectable2')]
+            }))
+          ]);
+        }).toThrowError('Multiple directives defined the same host injectable: "injectable1"');
       });
 
     });
+  });
 
   describe("ElementInjector", () => {
     describe("instantiate", () => {