feat(di): add support for optional dependencies

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

@@ -5,7 +5,7 @@ import {ProtoElementInjector, PreBuiltObjects, DirectiveBinding} from 'angular2/
 import {Parent, Ancestor} from 'angular2/src/core/annotations/visibility';
 import {EventEmitter} from 'angular2/src/core/annotations/events';
 import {onDestroy} from 'angular2/src/core/annotations/annotations';
-import {Injector, Inject, bind} from 'angular2/di';
+import {Optional, Injector, Inject, bind} from 'angular2/di';
 import {View} from 'angular2/src/core/compiler/view';
 import {ViewContainer} from 'angular2/src/core/compiler/view_container';
 import {NgElement} from 'angular2/src/core/dom/element';
@@ -36,6 +36,13 @@ class NeedsDirective {
   }
 }
 
+class OptionallyNeedsDirective {
+  dependency:SimpleDirective;
+  constructor(@Optional() dependency:SimpleDirective){
+    this.dependency = dependency;
+  }
+}
+
 class NeedDirectiveFromParent {
   dependency:SimpleDirective;
   constructor(@Parent() dependency:SimpleDirective){
@@ -342,6 +349,12 @@ export function main() {
             toThrowError('No provider for SimpleDirective! (NeedDirectiveFromParent -> SimpleDirective)');
       });
 
+      it("should inject null when no directive found", function () {
+        var inj = injector([OptionallyNeedsDirective]);
+        var d = inj.get(OptionallyNeedsDirective);
+        expect(d.dependency).toEqual(null);
+      });
+
       it("should accept SimpleDirective bindings instead of SimpleDirective types", function () {
         var inj = injector([
           DirectiveBinding.createFromBinding(bind(SimpleDirective).toClass(SimpleDirective), null)