Manage null CallSite receivers in script-inset

runtime/observatory/lib/src/elements/script_inset.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.
 
 library script_inset_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:observatory/app.dart';
 import 'package:observatory/models.dart' as M;
@@ skipping 1163 matching lines @@
         details.append(row('Call of "${callSite.name}" did not execute'));
       } else {
         var r = row();
         r.append(cell("Container"));
         r.append(cell("Count"));
         r.append(cell("Target"));
         details.append(r);
 
         for (var entry in callSite.entries) {
           var r = row();
-          r.append(cell(serviceRef(entry.receiver)));
+          if (entry.receiver == null) {
+            r.append(cell(""));
+          } else {
+            r.append(cell(serviceRef(entry.receiver)));
+          }
           r.append(cell(entry.count.toString()));
           r.append(cell(serviceRef(entry.target)));
           details.append(r);
         }
       }
       return details;
     });
   }
 }
 
@@ skipping 142 matching lines @@
     if (self) {
       return selfTicks / sampleCount;
     } else {
       return totalTicks / sampleCount;
     }
   }
 
   bool isHot(bool self) => _percent(self) > kHotThreshold;
   bool isMedium(bool self) => _percent(self) > kMediumThreshold;
 }