Fix 19029: handle special case when on-handler is empty in polymer, add better

pkg/polymer/lib/src/build/script_compactor.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.
 
 /// Transfomer that combines multiple dart script tags into a single one.
 library polymer.src.build.script_compactor;
 
 import 'dart:async';
 import 'dart:convert';
 
@@ skipping 496 matching lines @@
       }
       for (var exp in bindings.expressions) {
         _addExpression(exp, isEvent, isTwoWay);
       }
     });
   }
 
   void _addExpression(String stringExpression, bool inEvent, bool isTwoWay) {
     if (inEvent) {
       if (!stringExpression.startsWith("@")) {
+        if (stringExpression == '') return;
         generator.addGetter(stringExpression);
         generator.addSymbol(stringExpression);
         return;
       }
       stringExpression = stringExpression.substring(1);
     }
     visitor.run(pe.parse(stringExpression), isTwoWay);
   }
 }
 
@@ skipping 192 matching lines @@
   for (var c in combinators) {
     if (c is ShowElementCombinator) {
       var show = c.shownNames.toSet();
       elements.retainWhere((e) => show.contains(e.displayName));
     } else if (c is HideElementCombinator) {
       var hide = c.hiddenNames.toSet();
       elements.removeWhere((e) => hide.contains(e.displayName));
     }
   }
 }