Fix warnings in dart2js.

dart/lib/compiler/implementation/tree/nodes.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.
 
 abstract class Visitor<R> {
   const Visitor();
 
   abstract R visitNode(Node node);
 
   R visitBlock(Block node) => visitStatement(node);
@@ skipping 1617 matching lines @@
 
   Token getBeginToken() => beginToken;
 
   Token getEndToken() => endToken;
 
   LibraryTag toLibraryTag() {
     if (isImport()) {
       Identifier prefixNode;
       if (prefix !== null) {
         SourceString source = prefix.dartString.source;
+        Token prefixToken = prefix.getBeginToken();
         Token token = new StringToken.fromSource(IDENTIFIER_INFO, source,
-                                                 prefix.token.charOffset);
-        token.next = prefix.token.next;
+                                                 prefixToken.charOffset);
+        token.next = prefixToken.next;
         prefixNode = new Identifier(token);
       }
       return new Import(tag.token, argument, prefixNode, null);
     } else if (isLibrary()) {
       return new LibraryName(tag.token, argument);
     } else if (isSource()) {
       return new Part(tag.token, argument);
     } else {
       throw 'Unknown script tag ${tag.token.slowToString()}';
     }
@@ skipping 325 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send !== null) return f(send.arguments.head);
 }