Implement support for single stepping out of an async function.

runtime/vm/debugger.h

Index: runtime/vm/debugger.h
diff --git a/runtime/vm/debugger.h b/runtime/vm/debugger.h
index 63de2f5a8eb04449823f520dd128a77409da83a6..9bf4d969dfa8ffd4eb96a81274ec0ac9c8b92a34 100644
--- a/runtime/vm/debugger.h
+++ b/runtime/vm/debugger.h
@@ -361,6 +361,8 @@ class ActivationFrame : public ZoneAllocated {
   RawObject* GetAsyncCompleter();
   void ExtractTokenPositionFromAsyncClosure();
 
+  bool IsAsyncMachinery() const;
+
   static const char* KindToCString(Kind kind) {
     switch (kind) {
       case kRegular:
@@ -715,6 +717,12 @@ class Debugger {
                               const Code& code,
                               intptr_t post_deopt_frame_index);
 
+  void ResetSteppingFramePointers();
+  bool SteppedForSyntheticAsyncBreakpoint() const;
+  void CleanupSyntheticAsyncBreakpoint();
+  void RememberTopFrameAwaiter();
+  void SetAsyncSteppingFramePointer();
+
   Isolate* isolate_;
   Dart_Port isolate_id_;  // A unique ID for the isolate in the debugger.
   bool initialized_;
@@ -754,6 +762,9 @@ class Debugger {
   // frame corresponds to this fp value, or if the top frame is
   // lower on the stack.
   uword stepping_fp_;
+  // Used to track the current async/async* function.
+  uword async_stepping_fp_;
+  RawObject* top_frame_awaiter_;
 
   // If we step while at a breakpoint, we would hit the same pc twice.
   // We use this field to let us skip the next single-step after a