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";

[sra1, 2015/10/06 21:42:18]

Add a few more tests in the dart2js_native directory.
You can execute some setup code in that directory to create global JS things.

1. Minimal test that JS calling toString via a coercion calls Dart's toString.

2. Methods on @Js classes are not visible on Dart corelib intercepted classes.
You should not be able to call charCodeAt.

    @Js()
    class Foo {
      external charCodeAt(int i);
    }

    main() {
      var s = confuse('Hello');
      Expect.throws(() => s.charCodeAt(0));
    }
      

[Jacob, 2015/10/13 01:19:24]

Done. However put tests in test/html with the other interop tests as the tests
should run in dartium and ddc as well.
Put the confuse test in the same file as the existing test as that should just
increase the "confusion" :)
Kept the toString test separate to ensure we are retaining toString, etc.

+
+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();
 }