Support user generated custom native JS classes.

tests/compiler/dart2js_extra/js_interop_error_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
+import "package:js/js.dart";
+
+class Bar { }
+
+@Js()
+class Foo extends Bar {
+  external get bar;
+}
+
+@Js()
+class Baz {
+  Baz() {}
+  external get foo;
+  void bar() { }
+  get baz;
+}
 
 void main() {
-  int x = 327680;
-  int r = 65536;
-  for (var i = 0; i < 200; i++) {
-    Expect.equals(r, x ~/ 5);
-    x *= 10;
-    r *= 10;
-  }
+  /// 01: compile-time error
+  var b = new Bar();
 }