Support user generated custom native JS classes.

tests/html/js_dart_to_string_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.
+
+@Js()
+library js_typed_interop_test;
+
+import 'dart:html';
+
+import 'package:js/js.dart';
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+
+_injectJs() {
+  document.body.append(new ScriptElement()
+    ..type = 'text/javascript'
+    ..innerHtml = r"""
+
+  function jsToStringViaCoercion(a) {
+    return a + '';
+  };
+""");
+}
+
+@Js()
+external String jsToStringViaCoercion(obj);
+
+class ExampleClassWithCustomToString {
+  var x;
+  ExampleClassWithCustomToString(this.x);
+  String toString() => "#$x#";
+}
+
+main() {
+  _injectJs();
+
+  useHtmlConfiguration();
+
+  group('toString', () {
+    test('custom dart', () {
+      var x = new ExampleClassWithCustomToString("fooBar");
+      expect(jsToStringViaCoercion(x), equals("#fooBar#"));
+      expect(jsToStringViaCoercion({'a' : 1, 'b': 2}), equals("{a: 1, b: 2}"));
+    });
+  });
+}