Eliminating some dart2js warnings from polymer_expressions

pkg/polymer_expressions/lib/tokenizer.dart

Index: pkg/polymer_expressions/lib/tokenizer.dart
diff --git a/pkg/polymer_expressions/lib/tokenizer.dart b/pkg/polymer_expressions/lib/tokenizer.dart
index 4e03167f0b75972675a6301ca615dac3a66f9ad2..6d3f2c928ad23a3f8fbb23f3b83911d7e9f010fe 100644
--- a/pkg/polymer_expressions/lib/tokenizer.dart
+++ b/pkg/polymer_expressions/lib/tokenizer.dart
@@ -132,11 +132,11 @@ 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;
   }
 }