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

runtime/observatory/tests/service/service_test_common.dart

Index: runtime/observatory/tests/service/service_test_common.dart
diff --git a/runtime/observatory/tests/service/service_test_common.dart b/runtime/observatory/tests/service/service_test_common.dart
index 2a68a528c6b3be88ffd91c24c2f284ea7fa2a59e..243b41a2c4e9583e4d2923ed9ac101e54cae6098 100644
--- a/runtime/observatory/tests/service/service_test_common.dart
+++ b/runtime/observatory/tests/service/service_test_common.dart
@@ -234,6 +234,10 @@ 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'));