Fix stepping over await statements

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 "vm/code_generator.h"
 #include "vm/code_patcher.h"
@@ skipping 353 matching lines @@
     bpt->SetIsSingleShot();
     AddBreakpoint(bpt, dbg);
   }
   return bpt;
 }
 
 
 Breakpoint* BreakpointLocation::AddPerClosure(Debugger* dbg,
                                               const Instance& closure,
                                               bool for_over_await) {
-  Breakpoint* bpt = breakpoints();
-  while (bpt != NULL) {
-    if (bpt->IsPerClosure() && bpt->closure() == closure.raw()) break;
-    bpt = bpt->next();
+  Breakpoint* bpt = NULL;
+  // Do not reuse existing breakpoints for stepping over await clauses.
+  // A second async step-over command will set a new breakpoint before
+  // the existing one gets deleted when first async step-over resumes.
+  if (!for_over_await) {
+    bpt = breakpoints();
+    while (bpt != NULL) {
+      if (bpt->IsPerClosure() && (bpt->closure() == closure.raw())) break;
+      bpt = bpt->next();
+    }
   }
   if (bpt == NULL) {
     bpt = new Breakpoint(dbg->nextId(), this);
     bpt->SetIsPerClosure(closure);
     bpt->set_is_synthetic_async(for_over_await);
     AddBreakpoint(bpt, dbg);
   }
   return bpt;
 }
 
@@ skipping 2938 matching lines @@
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart