fix(dart): @proxy is a value, not a factory

Previously I added parens everywhere to make this @proxy() because our typing indicated
it was a function that returned a decorator, but this breaks dart. Instead, the typing needs
to be changed.

Fixes #3494

modules/angular2/src/mock/location_mock.ts

@@ -6,7 +6,7 @@ import {List, ListWrapper} from 'angular2/src/facade/collection';
 import {Location} from 'angular2/src/router/location';
 
 
-@proxy()
+@proxy
 @IMPLEMENTS(Location)
 export class SpyLocation extends SpyObject {
   urlChanges: List<string>;