Add support for javascript incompatibility warnings.

runtime/lib/errors_patch.dart

Index: runtime/lib/errors_patch.dart
===================================================================
--- runtime/lib/errors_patch.dart	(revision 35921)
+++ runtime/lib/errors_patch.dart	(working copy)
@@ -334,3 +334,11 @@
   _JavascriptIntegerOverflowError(this._value);
   String toString() => "Javascript Integer Overflow: $_value";
 }
+
+class _JavascriptCompatibilityError extends Error {
+  final String _msg;
+
+  _JavascriptCompatibilityError(this._msg);
+  String toString() => _msg;

[srdjan, 2014/05/08 18:11:05]

Maybe: "Javascript Compatibility Error: $_msg"; Then you could eliminate the
'javascript compatibility' text in many cases.

[regis, 2014/05/09 21:03:42]

Done.

+}
+