Remove Field::FindFieldIndex() and update service protocol

runtime/observatory/tests/service/get_object_rpc_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 library get_object_rpc_test;
 
 import 'dart:typed_data';
 import 'dart:convert' show BASE64;
 import 'package:observatory/service_io.dart';
@@ skipping 756 matching lines @@
       expect(e.message,
              startsWith("getObject: invalid 'objectId' parameter: "));
     }
     expect(caughtException, isTrue);
   },
 
   // field
   (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,
     };
     var result = await isolate.invokeRpcNoUpgrade('getObject', params);
     expect(result['type'], equals('Field'));
     expect(result['id'], equals(id));
     expect(result['name'], equals('dummyVar'));
     expect(result['const'], equals(false));
     expect(result['static'], equals(true));
     expect(result['final'], equals(false));
     expect(result['location']['type'], equals('SourceLocation'));
     expect(result['staticValue']['valueAsString'], equals('11'));
     expect(result['_guardNullable'], isNotNull);
     expect(result['_guardClass'], isNotNull);
     expect(result['_guardLength'], isNotNull);
   },
 
   // invalid field.
   (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,
     };
     bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getObject', params);
       expect(false, isTrue, reason:'Unreachable');
     } on ServerRpcException catch(e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
@@ skipping 44 matching lines @@
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
       expect(e.message,
              "getObject: invalid 'objectId' parameter: code/0");
     }
     expect(caughtException, isTrue);
   },
 ];
 
 main(args) async => runIsolateTests(args, tests, testeeBefore:warmup);