Remove Field::FindFieldIndex() and update service protocol

runtime/observatory/tests/service/get_object_rpc_test.dart

Index: runtime/observatory/tests/service/get_object_rpc_test.dart
diff --git a/runtime/observatory/tests/service/get_object_rpc_test.dart b/runtime/observatory/tests/service/get_object_rpc_test.dart
index 882b016cd2cd137e896a3a806027886d154faa45..6bfa71fdeff2c77f8f3e5e692570500ed89a57de 100644
--- a/runtime/observatory/tests/service/get_object_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_object_rpc_test.dart
@@ -774,7 +774,7 @@ var tests = [
   (Isolate isolate) async {
     // Call eval to get a class id.
     var evalResult = await eval(isolate, 'new _DummyClass()');
-    var id = "${evalResult['class']['id']}/fields/0";
+    var id = "${evalResult['class']['id']}/fields/dummyVar";
     var params = {
       'objectId': id,
     };
@@ -796,7 +796,7 @@ var tests = [
   (Isolate isolate) async {
     // Call eval to get a class id.
     var evalResult = await eval(isolate, 'new _DummyClass()');
-    var id = "${evalResult['class']['id']}/fields/9999";
+    var id = "${evalResult['class']['id']}/fields/mythicalField";
     var params = {
       'objectId': id,
     };