Add command-line parameter enabling assert messages.

tests/compiler/dart2js/parser_helper.dart

 // Copyright (c) 2012, 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 parser_helper;
 
 import "package:expect/expect.dart";
 
 import "package:compiler/src/elements/elements.dart";
 import "package:compiler/src/tree/tree.dart";
@@ skipping 107 matching lines @@
                         [void registerSource(Uri uri, String source)]) {
   Token tokens = scan(text);
   Uri uri = new Uri(scheme: "source", path: '${++sourceCounter}');
   if (registerSource != null) {
     registerSource(uri, text);
   }
   var script = new Script(uri, uri, new MockFile(text));
   var unit = new CompilationUnitElementX(script, library);
   int id = 0;
   ElementListener listener = new ElementListener(compiler, unit, () => id++);
-  PartialParser parser = new PartialParser(listener);
+  PartialParser parser =
+      new PartialParser(listener,
+                        allowAssertMessage: compiler.allowAssertMessage);
   compiler.withCurrentElement(unit, () => parser.parseUnit(tokens));
   return unit.localMembers;
 }
 
 NodeList fullParseUnit(String source, {DiagnosticListener diagnosticHandler}) {
   return parseBodyCode(source, (parser, tokens) => parser.parseUnit(tokens),
                        diagnosticHandler: diagnosticHandler);
 }