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

runtime/bin/vmservice/observatory/deployed/web/packages/unittest/coverage_controller.js

 // Copyright (c) 2011, 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.
 
 /**
  * Coverage controller logic - used by coverage test harness to embed tests in
  * content shell and extract coverage information.
  */
 
 var LONG_LINE = 60000;
@@ skipping 11 matching lines @@
     window.document.body.innerHTML = res;
     window.layoutTestController.notifyDone();
   }
 }
 
 if (window.layoutTestController) {
   window.layoutTestController.dumpAsText();
   window.layoutTestController.waitUntilDone();
   window.addEventListener("message", onReceive, false);
 }