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

Index: runtime/bin/vmservice/observatory/lib/src/elements/instance_ref.dart
diff --git a/runtime/bin/vmservice/observatory/lib/src/elements/instance_ref.dart b/runtime/bin/vmservice/observatory/lib/src/elements/instance_ref.dart
index 4fddfbb5a3c2ca74357e13223b7aded1be8e112a..4ea6822f9895d0965a921fd3b83ab1fd86d77b7f 100644
--- a/runtime/bin/vmservice/observatory/lib/src/elements/instance_ref.dart
+++ b/runtime/bin/vmservice/observatory/lib/src/elements/instance_ref.dart
@@ -37,20 +37,20 @@ class InstanceRefElement extends ServiceRefElement {
   }
 
   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();
     }
   }