Fix stepping in when paused in async functions.

runtime/vm/debugger.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/debugger.h"
 
 #include "include/dart_api.h"
 
 #include "platform/address_sanitizer.h"
 
@@ skipping 3261 matching lines @@
     top_frame_awaiter_ = stack_trace_->FrameAt(0)->GetAsyncAwaiter();
   } else {
     top_frame_awaiter_ = Object::null();
   }
 }
 
 void Debugger::SetAsyncSteppingFramePointer() {
   if (!FLAG_async_debugger) {
     return;
   }
-  if (stack_trace_->FrameAt(0)->function().IsAsyncClosure() ||
-      stack_trace_->FrameAt(0)->function().IsAsyncGenClosure()) {
+  if ((stack_trace_->Length()) > 0 &&
+      (stack_trace_->FrameAt(0)->function().IsAsyncClosure() ||
+       stack_trace_->FrameAt(0)->function().IsAsyncGenClosure())) {
     async_stepping_fp_ = stack_trace_->FrameAt(0)->fp();
   } else {
     async_stepping_fp_ = 0;
   }
 }
 
 void Debugger::HandleSteppingRequest(DebuggerStackTrace* stack_trace,
                                      bool skip_next_step) {
   ResetSteppingFramePointers();
   RememberTopFrameAwaiter();
@@ skipping 1076 matching lines @@
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart