Eliminating some dart2js warnings from polymer_expressions

pkg/polymer_expressions/lib/tokenizer.dart

 // Copyright (c) 2013, 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.
 
 library polymer_expressions.tokenizer;
 
 const int _TAB = 9;
 const int _LF = 10;
 const int _VTAB = 11;
 const int _FF = 12;
@@ skipping 114 matching lines @@
 bool isQuote(int next) => next == _DQ || next == _SQ;
 
 bool isNumber(int next) => _0 <= next && next <= _9;
 
 bool isOperator(int next) => _OPERATORS.contains(next);
 
 bool isGrouper(int next) => _GROUPERS.contains(next);
 
 int escape(int c) {
   switch (c) {
-    case _f: return _FF; break;
-    case _n: return _LF; break;
-    case _r: return _CR; break;
-    case _t: return _TAB; break;
-    case _v: return _VTAB; break;
+    case _f: return _FF;
+    case _n: return _LF;
+    case _r: return _CR;
+    case _t: return _TAB;
+    case _v: return _VTAB;
     default: return c;
   }
 }
 
 class Token {
   final int kind;
   final String value;
   final int precedence;
 
   Token(this.kind, this.value, [this.precedence = 0]);
@@ skipping 141 matching lines @@
     _tokens.add(new Token(GROUPER_TOKEN, value, _PRECEDENCE[value]));
     _advance();
   }
 }
 
 class ParseException implements Exception {
   final String message;
   ParseException(this.message);
   String toString() => "ParseException: $message";
 }