Fix bug in how we handle invoke expressions

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 522 matching lines @@
     super.visitIdentifier(e);
   }
 
   visitGetter(pe.Getter e) {
     _add(e.name);
     super.visitGetter(e);
   }
 
   visitInvoke(pe.Invoke e) {
     _includeSetter = false; // Invoke is only valid as an r-value.
-    _add(e.method);
+    if (e.method != null) _add(e.method);
     super.visitInvoke(e);
   }
 }
 
 /// Parses and collects information about bindings found in polymer templates.
 class _Mustaches {
   /// Each expression that appears within `{{...}}` and `[[...]]`.
   final List<String> expressions;
 
   /// Whether the whole text returned by [parse] was a single expression.
@@ skipping 135 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));
     }
   }
 }