Fail gracefully when attempting to evaluate against a VM internal class such a Field.

runtime/observatory/tests/service/eval_internal_class_test.dart

+// Copyright (c) 2016, 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.
+
+import 'dart:async';
+import 'dart:developer';
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+import 'service_test_common.dart';
+import 'test_helper.dart';
+
+var tests = [
+
+(Isolate isolate) async {
+  Library root = await isolate.rootLibrary.load();
+
+  Class classLibrary = await root.clazz.load();
+  print(classLibrary);
+  var result = await classLibrary.evaluate('3 + 4');
+  print(result);
+  expect(result is DartError, isTrue);
+  expect(result.message, contains('Cannot evaluate'));
+
+  Class classClass = await classLibrary.clazz.load();
+  print(classClass);
+  result = await classClass.evaluate('3 + 4');
+  print(result);
+  expect(result is DartError, isTrue);
+  expect(result.message, contains('Cannot evaluate'));
+
+  var someArray = await root.evaluate("new List(2)");
+  print(someArray);
+  expect(someArray is Instance, isTrue);
+  Class classArray = await someArray.clazz.load();
+  print(classArray);
+  result = await classArray.evaluate('3 + 4');
+  print(result);
+  expect(result is Instance, isTrue);
+  expect(result.valueAsString, equals('7'));
+},
+
+];
+
+main(args) => runIsolateTests(args, tests);