Give instances their own model class; move DartErrors out of instance-ref into their own error-ref.

runtime/bin/vmservice/observatory/lib/src/elements/instance_ref.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 instance_ref_element;
 
 import 'package:polymer/polymer.dart';
 import 'package:observatory/service.dart';
 import 'service_ref.dart';
 
@@ skipping 19 matching lines @@
     }
     return super.hoverText;
   }
 
   // TODO(turnidge): This is here to workaround vm/dart2js differences.
   dynamic expander() {
     return expandEvent;
   }
 
   void expandEvent(bool expand, var done) {
-    assert(ref is ServiceMap);
+    assert(ref is Instance);
     if (expand) {
       ref.reload().then((result) {
-        if (result['valueAsString'] != null) {
-          result.name = result['valueAsString'];
-          result.vmName = result['valueAsString'];
+        if (result.valueAsString != null) {
+          result.name = result.valueAsString;
+          result.vmName = result.valueAsString;
         }
         ref = result;
         notifyPropertyChange(#ref, 0, 1);
       }).whenComplete(done);
     } else {
-      ServiceMap refMap = ref;
-      refMap['fields'] = null;
-      refMap['elements'] = null;
+      Instance refMap = ref;
+      refMap.fields = null;
+      refMap.elements = null;
       done();
     }
   }
 }