Allow @JS on class method

Unlike top level function putting `@JS` on a class method has no effect.
A use case where it is needed is to make a Js member named `_m1()` on `A` be used outside of the current dart library.

``` dart
@JS
class A {
  external A();
  external _m1();
} 
```

I'd like to be able to have:

``` dart
@JS
class A {
  external A();
  @JS('_m1') external m1();
} 
```
