Add pause/resume for isolates in vmservice/observatory.

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 3fd21df97db5f576021cb6e5c7e6954baffaa6f6..aa40fe5c2eb844b1c942db6c2993bc1a4a233942 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1275,10 +1275,10 @@ DART_EXPORT bool Dart_HandleServiceMessages() {
   Isolate* isolate = Isolate::Current();
   CHECK_ISOLATE_SCOPE(isolate);
   CHECK_CALLBACK_STATE(isolate);
+
+  ASSERT(isolate->GetAndClearResumeRequest() == false);
   isolate->message_handler()->HandleOOBMessages();
-  // TODO(turnidge): The return value here should indicate whether an
-  // OOB message should cause the program to resume.  Implement.
-  return false;
+  return isolate->GetAndClearResumeRequest();
 }