Eliminating some dart2js warnings from polymer_expressions

pkg/polymer_expressions/lib/parser.dart

Index: pkg/polymer_expressions/lib/parser.dart
diff --git a/pkg/polymer_expressions/lib/parser.dart b/pkg/polymer_expressions/lib/parser.dart
index c02d656c5edfc7c8b182ae15ea7b43b33ce1f746..cc066816b9da658838f9ce4581e22975d66a6ca2 100644
--- a/pkg/polymer_expressions/lib/parser.dart
+++ b/pkg/polymer_expressions/lib/parser.dart
@@ -134,22 +134,16 @@ class Parser {
           return _astFactory.identifier('this');
         } else if (keyword == 'in') {
           return null;
-        } else {
-          throw new ArgumentError('unrecognized keyword: $keyword');
         }
-        break;
+        throw new ArgumentError('unrecognized keyword: $keyword');
       case IDENTIFIER_TOKEN:
         return _parseInvokeOrIdentifier();
-        break;
       case STRING_TOKEN:
         return _parseString();
-        break;
       case INTEGER_TOKEN:
         return _parseInteger();
-        break;
       case DECIMAL_TOKEN:
         return _parseDecimal();
-        break;
       case GROUPER_TOKEN:
         if (_token.value == '(') {
           return _parseParenthesized();
@@ -157,7 +151,6 @@ class Parser {
           return _parseMapLiteral();
         }
         return null;
-        break;
       default:
         return null;
     }