Fix warnings in dart2js.

sdk/lib/_internal/compiler/implementation/scanner/scanner.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.
 
 part of scanner;
 
 abstract class Scanner {
   Token tokenize();
 
   factory Scanner(SourceFile file, {bool includeComments: false}) {
@@ skipping 410 matching lines @@
       return next;
     }
   }
 
   int tokenizeOpenSquareBracket(int next) {
     // [ [] []=
     next = advance();
     if (identical(next, $CLOSE_SQUARE_BRACKET)) {
       Token token = previousToken();
       if (token is KeywordToken &&
-          identical((token as KeywordToken).keyword.syntax, 'operator')) {
+          identical(token.keyword.syntax, 'operator')) {
         return select($EQ, INDEX_EQ_INFO, INDEX_INFO);
       }
     }
     appendBeginGroup(OPEN_SQUARE_BRACKET_INFO);
     return next;
   }
 
   int tokenizeCaret(int next) {
     // ^ ^=
     return select($EQ, CARET_EQ_INFO, CARET_INFO);
@@ skipping 618 matching lines @@
     // ignore the [close] token (assuming it's correct), then the error will be
     // reported when parsing the [next] token.
 
     Token next = new StringToken.fromString(
         BAD_INPUT_INFO, error, begin.charOffset, canonicalize: true);
     begin.endGroup = close;
     close.next = next;
     next.next = begin.next;
   }
 }