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/test/test_lib/test_lib_spec.ts

@@ -21,7 +21,7 @@ class TestObj {
   someComplexFunc(a) { return a; }
 }
 
-@proxy()
+@proxy
 @IMPLEMENTS(TestObj)
 class SpyTestObj extends SpyObject {
   constructor() { super(TestObj); }