Support user generated custom native JS classes.

tests/html/json_helper.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.
 
-// Regression test for http://dartbug.com/22917
+@JsName("JSON")
+library json_helper;
 
-import 'package:expect/expect.dart';
+// TODO(jacobr): depend on package:js instead of defining this annotation here.

[sra1, 2015/09/17 19:52:57]

dart2js needs to find a known definition that is maintained in step with 

We could have a 'js interop annotations' library in js_runtime that contains
just the declaration of the annotation.  dart2js can find this definition just
like it does for other 'special' elements.

This can be re-exported by package:js

[Siggi Cherem (dart-lang), 2015/09/18 20:34:10]

how can package:js access this internal library? would it have to be a
configuration-specific package that only works in dart2js?

What if we do something similar to what we did for package:lookup_map?

[Jacob, 2015/10/01 00:47:34]

thanks for the link to what you did for lookup_map! I've duplicated what you did
for that package and moved "0.6.0" of package:js into the repo. I didn't bother
with a lot of complex versioning checks as the package:js only contains
annotations and not actual implementation.

[Siggi Cherem (dart-lang), 2015/10/01 18:21:28]

Makes sense you don't need to validate the version of the entire package. Let's
see if there is a way you can use the constant rather than trying to parse out
the annotation from the syntax. If we can manage that, it would make sense to
also add some logic to validate that the annotation looks correct (e.g. check
that the `name` field exists, otherwise, issue an error).

[Jacob, 2015/10/02 20:08:15]

got any pointers for how that should be done? There are other annotations such
as @Native that it would be better if dart2js evaluated instead of just parsing
so I'd rather not block this CL on that. After this lands I can start working on
a followup CL to reduce the technical debt processing annotations.

+class JsName {
+  const JsName([this.name]);
+  final String name;
+}
 
-m(x) => print('x: $x');
-    
-test() => Function.apply(m, []);
-
-main() {
-  Expect.throws(test);
-}
+external String stringify(object);