feat(di): introduce aliasing

Closes #710
Closes #747

modules/angular2/docs/di/di.md

@@ -96,7 +96,7 @@ var car = child.get(Car); // uses the Car binding from the parent injector and E
 
 ## Bindings
 
-You can bind to a class, a value, or a factory
+You can bind to a class, a value, or a factory. It is also possible to alias existing bindings.
 
 ```
 var inj = new Injector([
@@ -128,6 +128,16 @@ var inj = new Injector([
 ]);
 ```
 
+If you want to alias an existing binding, you can do so using `toAlias`:
+
+```
+var inj = new Injector([
+	bind(Engine).toClass(Engine),
+	bind("engine!").toAlias(Engine)
+]);
+```
+which implies `inj.get(Engine) === inj.get("engine!")`.
+
 Note that tokens and factory functions are decoupled.
 
 ```

modules/angular2/src/di/binding.js

@@ -60,6 +60,15 @@ export class BindingBuilder {
     );
   }
 
+  toAlias(aliasToken):Binding {
+    return new Binding(
+      Key.get(this.token),
+      (aliasInstance) => aliasInstance,
+      [new Dependency(Key.get(aliasToken), false, false, [])],
+      false
+    );
+  }
+
   toFactory(factoryFunction:Function, dependencies:List = null):Binding {
     return new Binding(
       Key.get(this.token),

modules/angular2/test/di/injector_spec.js

@@ -128,6 +128,26 @@ export function main() {
       expect(car.engine).toBeAnInstanceOf(Engine);
     });
 
+    it('should bind to an alias', function() {
+      var injector = new Injector([
+        Engine,
+        bind(SportsCar).toClass(SportsCar),
+        bind(Car).toAlias(SportsCar)
+      ]);
+
+      var car = injector.get(Car);
+      var sportsCar = injector.get(SportsCar);
+      expect(car).toBeAnInstanceOf(SportsCar);
+      expect(car).toBe(sportsCar);
+    });
+
+    it('should throw when the aliased binding does not exist', function () {
+      var injector = new Injector([
+        bind('car').toAlias(SportsCar)
+      ]);
+      expect(() => injector.get('car')).toThrowError('No provider for SportsCar! (car -> SportsCar)');
+    });
+
     it('should support overriding factory dependencies', function () {
       var injector = new Injector([
         Engine,