Add command-line parameter enabling assert messages.

pkg/compiler/lib/src/scanner/parser_task.dart

 // Copyright (c) 2011, 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 scanner;
 
 class ParserTask extends CompilerTask {
   ParserTask(Compiler compiler) : super(compiler);
   String get name => 'Parser';
 
   Node parse(ElementX element) {
     return measure(() => element.parseNode(compiler));
   }
 
   Node parseCompilationUnit(Token token) {
     return measure(() {
       NodeListener listener = new NodeListener(compiler, null);
-      Parser parser = new Parser(listener);
+      Parser parser =
+          new Parser(listener, allowAssertMessage: compiler.allowAssertMessage);
       try {
         parser.parseUnit(token);
       } on ParserError catch(_) {
         assert(invariant(token, compiler.compilationFailed));
         return listener.makeNodeList(0, null, null, '\n');
       }
       Node result = listener.popNode();
       assert(listener.nodes.isEmpty);
       return result;
     });
   }
 }