Implement messages in assert() in dev_compiler.

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

 // 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.
 
 library dart._js_helper;
 
 import 'dart:collection';
 
 import 'dart:_foreign_helper' show
     JS,
@@ skipping 833 matching lines @@
 /**
  * Error thrown when a runtime error occurs.
  */
 class RuntimeError extends Error {
   final message;
   RuntimeError(this.message);
   String toString() => "RuntimeError: $message";
 }
 
 /**
+ * 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.
  */
 int random64() {
   // TODO(lrn): Use a secure random source.
   int int32a = JS("int", "(Math.random() * 0x100000000) >>> 0");
   int int32b = JS("int", "(Math.random() * 0x100000000) >>> 0");
   return int32a + int32b * 0x100000000;
 }
@@ skipping 30 matching lines @@
   // we have no way of telling the compiler yet, so it will generate an extra
   // layer of indirection that wraps the SyncIterator.
   _jsIterator() => JS('', '#(...#)', _generator, _args);
 
   Iterator<E> get iterator => new SyncIterator<E>(_jsIterator());
 }
 
 class BooleanConversionAssertionError extends AssertionError {
   toString() => 'Failed assertion: boolean expression must not be null';
 }