[observatory] Fix _guardLength serialization to output a string, rath…

…er than an int.

Change-Id: I12e3505649ebe9bb72f479dc7e2fc2dd19226ed7
Reviewed-on: https://dart-review.googlesource.com/68802
Reviewed-by: Ryan Macnak <rmacnak@google.com>

runtime/observatory/tests/service/get_object_rpc_test.dart

@@ -14,6 +14,7 @@ import 'test_helper.dart';
 
 class _DummyClass {
   static var dummyVar = 11;
+  final List<String> dummyList = new List<String>(20);
   void dummyFunction() {}
 }
 
@@ -884,6 +885,27 @@ var tests = <IsolateTest>[
     expect(result['_guardLength'], isNotNull);
   },
 
+  // field
+  (Isolate isolate) async {
+    // Call eval to get a class id.
+    var evalResult = await eval(isolate, 'new _DummyClass()');
+    var id = "${evalResult['class']['id']}/fields/dummyList";
+    var params = {
+      'objectId': id,
+    };
+    var result = await isolate.invokeRpcNoUpgrade('getObject', params);
+    expect(result['type'], equals('Field'));
+    expect(result['id'], equals(id));
+    expect(result['name'], equals('dummyList'));
+    expect(result['const'], equals(false));
+    expect(result['static'], equals(false));
+    expect(result['final'], equals(true));
+    expect(result['location']['type'], equals('SourceLocation'));
+    expect(result['_guardNullable'], isNotNull);
+    expect(result['_guardClass'], isNotNull);
+    expect(result['_guardLength'], equals('20'));
+  },
+
   // invalid field.
   (Isolate isolate) async {
     // Call eval to get a class id.

runtime/vm/object_service.cc

@@ -394,7 +394,7 @@ void Field::PrintJSONImpl(JSONStream* stream, bool ref) const {
   } else if (guarded_list_length() == kNoFixedLength) {
     jsobj.AddProperty("_guardLength", "variable");
   } else {
-    jsobj.AddProperty("_guardLength", guarded_list_length());
+    jsobj.AddPropertyF("_guardLength", "%" Pd, guarded_list_length());
   }
   const class Script& script = Script::Handle(Script());
   if (!script.IsNull()) {