Copy dart2js source into pkg/compiler

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

 // Copyright (c) 2014, 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 native;
 
 void checkAllowedLibrary(ElementListener listener, Token token) {
   LibraryElement currentLibrary = listener.compilationUnitElement.library;
   if (!currentLibrary.canUseNative) {
     listener.recoverableError(token, "Unexpected token");
@@ skipping 23 matching lines @@
     hasExpression = true;
     listener.beginLiteralString(token);
     listener.endLiteralString(0);
     token = token.next;
   }
   listener.endReturnStatement(hasExpression, begin, token);
   // TODO(ngeoffray): expect a ';'.
   // Currently there are method with both native marker and Dart body.
   return token.next;
 }