Fix evaluation in frames with _private variable names.

runtime/observatory/tests/service/evaluate_in_frame_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
+
+import 'dart:async';
+import 'dart:developer';
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+import 'test_helper.dart';
+
+void method(int value, _) {
+  debugger();
+}
+
+void testFunction() {
+  int i = 0;
+  while (true) {
+    if (++i % 100000000 == 0) {
+      method(10000, 50);
+    }
+  }
+}
+
+var tests = [
+
+hasStoppedAtBreakpoint,
+
+// Evaluate against library, class, and instance.
+(Isolate isolate) async {
+  var result;
+  result = await isolate.evalFrame(0, 'value');
+  expect(result.valueAsString, equals('10000'));
+
+  result = await isolate.evalFrame(0, '_');
+  expect(result.valueAsString, equals('50'));
+
+  result = await isolate.evalFrame(0, 'value + _');
+  expect(result.valueAsString, equals('10050'));
+
+  result = await isolate.evalFrame(1, 'i');
+  expect(result.valueAsString, equals('100000000'));
+},
+
+];
+
+main(args) => runIsolateTests(args, tests, testeeConcurrent: testFunction);