Disable awaiter stepping by default until context level computation is fixed

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 1937 matching lines @@
     }
     // Follow the link.
     async_stack_trace = async_stack_trace.async_link();
   }
 
   return stack_trace;
 }
 
 
 DebuggerStackTrace* Debugger::CollectAwaiterReturnStackTrace() {
-  if (!FLAG_causal_async_stacks) {
+  if (!FLAG_async_debugger_stepping) {
     return NULL;
   }
   // Causal async stacks are not supported in the AOT runtime.
   ASSERT(!FLAG_precompiled_runtime);
 
   Thread* thread = Thread::Current();
   Zone* zone = thread->zone();
   Isolate* isolate = thread->isolate();
   DebuggerStackTrace* stack_trace = new DebuggerStackTrace(8);
 
@@ skipping 2300 matching lines @@
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart