left + right mouse button causes dart exception

sky/sdk/lib/rendering/sky_binding.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky' as sky;
 
 import '../base/scheduler.dart' as scheduler;
 import '../base/hit_test.dart';
 import 'box.dart';
 import 'object.dart';
 
+int _hammingWeight(int value) {
+  if (value == 0)
+    return 0;
+  int weight = 0;
+  for (int i = 0; i < value.bitLength; ++i) {
+    if (value & (1 << i) != 0)
+      ++weight;
+  }
+  return weight;
+}
+
 class PointerState {
   PointerState({ this.result, this.lastPosition });
   HitTestResult result;
   Point lastPosition;
 }
 
 typedef void EventListener(sky.Event event);
 
 class SkyBinding {
 
@@ skipping 75 matching lines @@
     Point position = new Point(event.x, event.y);
 
     PointerState state;
     switch(event.type) {
       case 'pointerdown':
         state = _createStateForPointer(event, position);
         break;
       case 'pointerup':
       case 'pointercancel':
         state = _stateForPointer[event.pointer];
-        _stateForPointer.remove(event.pointer);
+        if (_hammingWeight(event.buttons) <= 1)
+          _stateForPointer.remove(event.pointer);
         break;
       case 'pointermove':
         state = _stateForPointer[event.pointer];
         // In the case of mouse hover we won't already have a cached down.
         if (state == null)
           state = _createStateForPointer(event, position);
         break;
     }
     event.dx = position.x - state.lastPosition.x;
     event.dy = position.y - state.lastPosition.y;
     state.lastPosition = position;
 
     dispatchEvent(event, state.result);
   }
 
   void dispatchEvent(sky.Event event, HitTestResult result) {
     assert(result != null);
     for (HitTestEntry entry in result.path.reversed)
       entry.target.handleEvent(event, entry);
   }
 
   String toString() => 'Render Tree:\n${_renderView}';
 
   void debugDumpRenderTree() {
     toString().split('\n').forEach(print);
   }
 
 }