Fix warnings in dart2js.

sdk/lib/_internal/compiler/implementation/js/printer.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 js;
 
 class Printer implements NodeVisitor {
   final bool shouldCompressOutput;
   leg.Compiler compiler;
   leg.CodeBuffer outBuffer;
@@ skipping 438 matching lines @@
     lineOut();
   }
 
   visitNestedExpression(Expression node, int requiredPrecedence,
                         {bool newInForInit, bool newAtStatementBegin}) {
     bool needsParentheses =
         // a - (b + c).
         (requiredPrecedence != EXPRESSION &&
          node.precedenceLevel < requiredPrecedence) ||
         // for (a = (x in o); ... ; ... ) { ... }
-        (newInForInit && node is Binary && (node as Binary).op == "in") ||
+        (newInForInit && node is Binary && node.op == "in") ||
         // (function() { ... })().
         // ({a: 2, b: 3}.toString()).
         (newAtStatementBegin && (node is NamedFunction ||
                                  node is Fun ||
                                  node is ObjectInitializer));
     if (needsParentheses) {
       inForInit = false;
       atStatementBegin = false;
       out("(");
       visit(node);
@@ skipping 671 matching lines @@
         codes.add(nthLetter((n ~/ nameSpaceSize) % LETTERS));
       }
       codes.add(charCodes.$0 + digit);
       newName = new String.fromCharCodes(codes);
     }
     assert(new RegExp(r'[a-zA-Z][a-zA-Z0-9]*').hasMatch(newName));
     maps.last[oldName] = newName;
     return newName;
   }
 }