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

runtime/bin/vmservice/observatory/lib/src/elements/error_ref.dart

Index: runtime/bin/vmservice/observatory/lib/src/elements/error_ref.dart
diff --git a/runtime/bin/vmservice/observatory/lib/src/elements/error_ref.dart b/runtime/bin/vmservice/observatory/lib/src/elements/error_ref.dart
new file mode 100644
index 0000000000000000000000000000000000000000..14d364e37b196cfc87fd325af3d86c22d5eba2c2
--- /dev/null
+++ b/runtime/bin/vmservice/observatory/lib/src/elements/error_ref.dart
@@ -0,0 +1,14 @@
+// Copyright (c) 2014, 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 error_ref_element;
+
+import 'package:polymer/polymer.dart';
+import 'package:observatory/service.dart';
+import 'service_ref.dart';
+
+@CustomTag('error-ref')
+class ErrorRefElement extends ServiceRefElement {
+  ErrorRefElement.created() : super.created();
+}