Implement rewind: drop one or more frames from the debugger.

runtime/vm/simulator_dbc.cc

 // Copyright (c) 2016, 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 <setjmp.h>  // NOLINT
 #include <stdlib.h>
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_DBC)
 
@@ skipping 3697 matching lines @@
 
   // Set the tag.
   thread->set_vm_tag(VMTag::kDartTagId);
   // Clear top exit frame.
   thread->set_top_exit_frame_info(0);
 
   sp_ = reinterpret_cast<RawObject**>(sp);
   fp_ = reinterpret_cast<RawObject**>(fp);
 
   if (pc == StubCode::RunExceptionHandler_entry()->EntryPoint()) {
-    // Instead of executing the RunException stub, we implement its
-    // behavior here.
+    // The RunExceptionHandler stub is a placeholder.  We implement
+    // its behavior here.
     RawObject* raw_exception = thread->active_exception();
     RawObject* raw_stacktrace = thread->active_stacktrace();
     ASSERT(raw_exception != Object::null());
     special_[kExceptionSpecialIndex] = raw_exception;
     special_[kStacktraceSpecialIndex] = raw_stacktrace;
     pc_ = thread->resume_pc();
+  } else if (pc == StubCode::DeoptForRewind_entry()->EntryPoint()) {
+    // The DeoptForRewind stub is a placeholder.  We will eventually
+    // implement its behavior here.
+    //
+    // TODO(turnidge): Refactor the Deopt bytecode so that we can use
+    // the implementation here too.  The deopt pc is stored in
+    // Thread::resume_pc().  After invoking deoptimization, we usually
+    // call into Debugger::RewindPostDeopt(), but I need to figure out
+    // if that makes any sense (it would JumpToFrame during a
+    // JumpToFrame, which seems wrong).
+    UNIMPLEMENTED();
   } else {
     pc_ = pc;
   }
 
   buf->Longjmp();
   UNREACHABLE();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_DBC