Fix crash when determining whether an async exception is handled

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 788 matching lines @@
   return Object::null();
 }
 
 
 RawObject* ActivationFrame::GetCausalStack() {
   return GetAsyncContextVariable(Symbols::AsyncStackTraceVar());
 }
 
 
 bool ActivationFrame::HandlesException(const Instance& exc_obj) {
+  if ((kind_ == kAsyncSuspensionMarker) || (kind_ == kAsyncCausal)) {
+    // These frames are historical.
+    return false;
+  }
   intptr_t try_index = TryIndex();
   if (try_index < 0) {
     return false;
   }
   ExceptionHandlers& handlers = ExceptionHandlers::Handle();
   Array& handled_types = Array::Handle();
   AbstractType& type = Type::Handle();
   const bool is_async =
       function().IsAsyncClosure() || function().IsAsyncGenClosure();
   handlers = code().exception_handlers();
@@ skipping 3426 matching lines @@
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart