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

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

Index: runtime/bin/vmservice/observatory/lib/src/elements/service_ref.dart
diff --git a/runtime/bin/vmservice/observatory/lib/src/elements/service_ref.dart b/runtime/bin/vmservice/observatory/lib/src/elements/service_ref.dart
index 466799dfe8f68f53ee6171e8b9a6b7fa0aa7e217..b75e6074b3a899fee9f0a4d00be53b078bca1237 100644
--- a/runtime/bin/vmservice/observatory/lib/src/elements/service_ref.dart
+++ b/runtime/bin/vmservice/observatory/lib/src/elements/service_ref.dart
@@ -66,7 +66,7 @@ class AnyServiceRefElement extends ObservatoryElement {
   Element _constructElementForRef() {
     var type = ref.serviceType;
     switch (type) {
-      case 'Class':
+     case 'Class':
         ServiceRefElement element = new Element.tag('class-ref');
         element.ref = ref;
         return element;
@@ -74,6 +74,10 @@ class AnyServiceRefElement extends ObservatoryElement {
         ServiceRefElement element = new Element.tag('code-ref');
         element.ref = ref;
         return element;
+      case 'Error':
+        ServiceRefElement element = new Element.tag('error-ref');
+        element.ref = ref;
+        return element;
       case 'Field':
         ServiceRefElement element = new Element.tag('field-ref');
         element.ref = ref;
@@ -95,6 +99,7 @@ class AnyServiceRefElement extends ObservatoryElement {
       case 'Instance':
       case 'Mint':
       case 'Null':
+      case 'Sentinel':  // TODO(rmacnak): Separate this out.
       case 'Smi':
       case 'String':
       case 'Type':