Implement rewind: drop one or more frames from the debugger.

runtime/observatory/tests/service/service_test_common.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.
 
 library service_test_common;
 
 import 'dart:async';
 import 'package:observatory/models.dart' as M;
 import 'package:observatory/service_common.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 216 matching lines @@
     print("Breakpoint is $bpt");
     expect(bpt, isNotNull);
     expect(bpt is Breakpoint, isTrue);
   };
 }
 
 IsolateTest stoppedAtLine(int line) {
   return (Isolate isolate) async {
     print("Checking we are at line $line");
 
+    // Make sure that the isolate has stopped.
+    isolate.reload();
+    expect(isolate.pauseEvent is! M.ResumeEvent, isTrue);
+
     ServiceMap stack = await isolate.getStack();
     expect(stack.type, equals('Stack'));
 
     List<Frame> frames = stack['frames'];
     expect(frames.length, greaterThanOrEqualTo(1));
 
     Frame top = frames[0];
     Script script = await top.location.script.load();
     int actualLine = script.tokenToLine(top.location.tokenPos);
     if (actualLine != line) {
@@ skipping 121 matching lines @@
 
 Future<Instance> rootLibraryFieldValue(Isolate isolate,
                                        String fieldName) async {
   Library rootLib = await isolate.rootLibrary.load();
   Field field = rootLib.variables.singleWhere((v) => v.name == fieldName);
   await field.load();
   Instance value = field.staticValue;
   await value.load();
   return value;
 }