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/test_lib/test_lib.ts

@@ -12,9 +12,7 @@ import {createTestInjector, FunctionWithParamTokens, inject} from './test_inject
 
 export {inject} from './test_injector';
 
-export function proxy(): ClassDecorator {
-  return (t) => t;
-}
+export var proxy: ClassDecorator = (t) => t;
 
 var _global: jasmine.GlobalPolluter = <any>(typeof window === 'undefined' ? global : window);