Implement messages in assert() in dev_compiler.

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart

 // Copyright (c) 2015, 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.
 part of dart._runtime;
 
 throwCastError(object, actual, type) => JS('', '''(() => {
   debugger;
   $throw_(new $CastErrorImplementation($object,
                                        $typeName($actual),
                                        $typeName($type)));
@@ skipping 18 matching lines @@
   $throw_(new $StrongModeTypeError($object,
                                    $typeName($actual),
                                    $typeName($type)));
 })()''');
 
 throwUnimplementedError(message) => JS('', '''(() => {
   debugger;
   $throw_(new $UnimplementedError($message));
 })()''');
 
-throwAssertionError() => JS('', '''(() => {
+throwAssertionError([message]) => JS('', '''(() => {
   debugger;
-  $throw_(new $AssertionError());
+  let error = $message != null
+        ? new $AssertionErrorWithMessage($message())
+        : new $AssertionError();
+  $throw_(error);
 })()''');
 
 throwNullValueError() => JS('', '''(() => {
   // TODO(vsm): Per spec, we should throw an NSM here.  Technically, we ought
   // to thread through method info, but that uglifies the code and can't
   // actually be queried ... it only affects how the error is printed.
   debugger;
   $throw_(new $NoSuchMethodError(null,
       new $Symbol('<Unexpected Null Value>'), null, null, null));
 })()''');