Reland improvements to causal async stack traces

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 1684 matching lines @@
   // frames but a historical record of how this asynchronous function was
   // activated.
   while (!async_stack_trace.IsNull()) {
     for (intptr_t i = 0; i < async_stack_trace.Length(); i++) {
       if (async_stack_trace.CodeAtFrame(i) == Code::null()) {
         break;
       }
       if (async_stack_trace.CodeAtFrame(i) ==
           StubCode::AsynchronousGapMarker_entry()->code()) {
         stack_trace->AddMarker(ActivationFrame::kAsyncSuspensionMarker);
+        // The frame immediately below the asynchronous gap marker is the
+        // identical to the frame above the marker. Skip the frame to enhance
+        // the readability of the trace.
+        i++;
       } else {
         code = Code::RawCast(async_stack_trace.CodeAtFrame(i));
         offset = Smi::RawCast(async_stack_trace.PcOffsetAtFrame(i));
         uword pc = code.PayloadStart() + offset.Value();
         if (code.is_optimized()) {
           for (InlinedFunctionsIterator it(code, pc); !it.Done();
                it.Advance()) {
             inlined_code = it.code();
             stack_trace->AddAsyncCausalFrame(pc, inlined_code);
           }
@@ skipping 2112 matching lines @@
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart