Give fields a model class in the service library.

runtime/bin/vmservice/observatory/test/inbound_references_test.dart

Index: runtime/bin/vmservice/observatory/test/inbound_references_test.dart
diff --git a/runtime/bin/vmservice/observatory/test/inbound_references_test.dart b/runtime/bin/vmservice/observatory/test/inbound_references_test.dart
index f515671caa55a1542909795280b21d9896886970..0d9698c5f400e92f87d31a31743280ac4bf423a8 100644
--- a/runtime/bin/vmservice/observatory/test/inbound_references_test.dart
+++ b/runtime/bin/vmservice/observatory/test/inbound_references_test.dart
@@ -30,7 +30,7 @@ var tests = [
 
 (Isolate isolate) =>
   isolate.rootLib.load().then((Library lib) {
-    Instance e = lib.variables.where((v) => v.name == 'e').single['value'];
+    Instance e = lib.variables.where((v) => v.name == 'e').single.value;
     var id = e.id;
     return isolate.get('/$id/inbound_references?limit=100').then(
         (ServiceMap response) {
@@ -41,15 +41,14 @@ var tests = [
 
           // Assert e is referenced by at least n, array, and the top-level
           // field e.
-          hasReferenceSuchThat((r) => r['slot'] is Map &&
-                                      r['slot']['type']=='@Field' &&
-                                      r['slot']['name']=='edge' &&
+          hasReferenceSuchThat((r) => r['slot'] is Field &&
+                                      r['slot'].name=='edge' &&
                                       r['source'].isInstance &&
                                       r['source'].clazz.name=='Node');
           hasReferenceSuchThat((r) => r['slot'] == 1 &&
                                       r['source'].isList);
           hasReferenceSuchThat((r) => r['slot']=='<unknown>' &&
-                                      r['source']['type']=='@Field');
+                                      r['source'] is Field);
     });
 }),