fix(core): workaround for typescript@1.7.3 breakage #5784

I don't understand why I need to declare the type-here, but it resolves the issue.

Looks like a bug in tsc.

Fixes #5784

modules/angular2/src/core/metadata/di.ts

@@ -32,7 +32,7 @@ import {DependencyMetadata} from 'angular2/src/core/di/metadata';
 export class AttributeMetadata extends DependencyMetadata {
   constructor(public attributeName: string) { super(); }
 
-  get token() {
+  get token(): AttributeMetadata {
     // Normally one would default a token to a type of an injected value but here
     // the type of a variable is "string" and we can't use primitive type as a return value
     // so we use instance of Attribute instead. This doesn't matter much in practice as arguments