Add optional message to assert in Dart2js.

sdk/lib/core/errors.dart

Index: sdk/lib/core/errors.dart
diff --git a/sdk/lib/core/errors.dart b/sdk/lib/core/errors.dart
index 0e99b0b271ce48375f613c2fc9522aad6d0e5019..5d6ac7fcc03ad3594f3daae878ee009a19643694 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -96,6 +96,11 @@ class Error {
  * Error thrown by the runtime system when an assert statement fails.
  */
 class AssertionError extends Error {
+  final Object message;
+  AssertionError([this.message]);
+  String toString() =>
+      "Assertion failed: ${message == null ? "is not true."
+                                           : Error.safeToString(message)}";
 }
 
 /**