Debugger support for step-into async and async* functions.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 5b9f0709e4f4d0a6c7fae06be76b97b3af152aa8..c7749e2db0cabb47618aa3bf3e90a13655039b5c 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1138,9 +1138,12 @@ bool Isolate::MakeRunnable() {
   ASSERT(object_store()->root_library() != Library::null());
   set_is_runnable(true);
 #ifndef PRODUCT
-  if (FLAG_support_debugger && !ServiceIsolate::IsServiceIsolate(this)) {
-    if (FLAG_pause_isolates_on_unhandled_exceptions) {
-      debugger()->SetExceptionPauseInfo(kPauseOnUnhandledExceptions);
+  if (FLAG_support_debugger) {
+    if (!ServiceIsolate::IsServiceIsolate(this)) {
+      debugger()->OnIsolateRunnable();
+      if (FLAG_pause_isolates_on_unhandled_exceptions) {
+        debugger()->SetExceptionPauseInfo(kPauseOnUnhandledExceptions);
+      }
     }
   }
 #endif  // !PRODUCT