Implement messages in assert() in dev_compiler.

pkg/dev_compiler/tool/input_sdk/private/js_helper.dart

Index: pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart b/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
index 351d94d0195a1078c545dbbe257777290b51ca05..336eaa20047ac2e9bea880b14bac082c08a1d96d 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
@@ -851,6 +851,17 @@ class RuntimeError extends Error {
 }
 
 /**
+ * Error thrown when an assert() fails with a message:
+ *
+ *     assert(false, "Message here");
+ */
+class AssertionErrorWithMessage extends AssertionError {
+  final Object _message;
+  AssertionErrorWithMessage(this._message);
+  String toString() => "Assertion failed: ${_message}";
+}
+
+/**
  * Creates a random number with 64 bits of randomness.
  *
  * This will be truncated to the 53 bits available in a double.