Add dartExperimentalFixupGetTag for polymer experiments.

tests/compiler/dart2js_native/fixup_get_tag_test.dart

Index: tests/compiler/dart2js_native/fixup_get_tag_test.dart
diff --git a/tests/compiler/dart2js_native/fixup_get_tag_test.dart b/tests/compiler/dart2js_native/fixup_get_tag_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..fbf9e3ad965cb2eade9e6c0b5f59a6449be66291
--- /dev/null
+++ b/tests/compiler/dart2js_native/fixup_get_tag_test.dart
@@ -0,0 +1,50 @@
+// Copyright (c) 2013, 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";
+
+// Test for dartExperimentalFixupGetTag.
+
+class Foo native "A" {  // There is one native class with dispatch tag 'A'.

[ahe, 2013/07/24 08:46:18]

Extract space before //.

[sra1, 2013/07/25 21:47:09]

Hmm.  I recall being told in a code review to put two spaces before the comment
but a quick search didn't find it.

+  token() native;
+}
+
+void setup() native r"""
+
+dartExperimentalFixupGetTag = function (originalGetTag) {
+  function fixedGetTag(obj) {
+    // Local JS 'class' B is made to be another implementation of the native
+    // class with tag 'A'.
+    if (obj instanceof B) return 'A';
+    // Other classes behave as before.
+    return originalGetTag(obj);
+  }
+  return fixedGetTag;
+};
+
+function A(){ }
+A.prototype.token = function () { return 'isA'; }
+
+function B(){ }
+B.prototype.token = function () { return 'isB'; }
+
+makeA = function() { return new A; };
+makeB = function() { return new B; };
+""";
+
+makeA() native;
+makeB() native;
+
+main() {
+  setup();
+
+  var a = makeA();
+  var b = makeB();
+
+  Expect.equals('isA', a.token());
+
+  // This call succeeds because the fixed-up 'getTag' method returns Foo's
+  // dispatch tag, and B is a faithful polyfil for Foo/A.
+  Expect.equals('isB', b.token());

[ahe, 2013/07/24 08:46:18]

Would it make sense to add:

Expect.isTrue(a is A);
Expect.isTrue(b is A);

[sra1, 2013/07/25 21:47:09]

Should be: Expect.isTrue(b is Foo);
Done.

+}