Add optional message argument to assert statements in the VM.

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 c11ea2e1a97da25e0c73490a216d7ec025492dc6..b6487a9dffebbce057267a7d014a6f01032f958e 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -96,7 +96,9 @@ class Error {
  * Error thrown by the runtime system when an assert statement fails.
  */
 class AssertionError extends Error {
-  AssertionError();
+  /** Message describing the assertion error. */
+  final Object message;
+  AssertionError([this.message]);
   String toString() => "Assertion failed";
 }