Reland https://code.google.com/p/dart/source/detail?r=29315:

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 846 matching lines @@
       src_breakpoints_(NULL),
       code_breakpoints_(NULL),
       resume_action_(kContinue),
       ignore_breakpoints_(false),
       in_event_notification_(false),
       exc_pause_info_(kNoPauseOnExceptions) {
 }
 
 
 Debugger::~Debugger() {
-  PortMap::ClosePort(isolate_id_);
   isolate_id_ = ILLEGAL_ISOLATE_ID;
   ASSERT(!in_event_notification_);
   ASSERT(src_breakpoints_ == NULL);
   ASSERT(code_breakpoints_ == NULL);
   ASSERT(stack_trace_ == NULL);
   ASSERT(obj_cache_ == NULL);
 }
 
 
 void Debugger::Shutdown() {
@@ skipping 1035 matching lines @@
     InstrumentForStepping(func_to_instrument);
   }
 }
 
 
 void Debugger::Initialize(Isolate* isolate) {
   if (initialized_) {
     return;
   }
   isolate_ = isolate;
-  // Create a port here, we don't expect to receive any messages on this port.
+  // Use the isolate's control port as the isolate_id for debugging.
   // This port will be used as a unique ID to represet the isolate in the
   // debugger wire protocol messages.
-  // NOTE: SetLive is never called on this port.
-  isolate_id_ = PortMap::CreatePort(isolate->message_handler());

[Ivan Posva, 2013/10/27 14:33:17]

Since the main port (aka control port) survives to the end of the isolate, we
can now reuse that port for debugger identification.

+  isolate_id_ = isolate->main_port();
   initialized_ = true;
 
   // Signal isolate creation event.
   SignalIsolateEvent(Debugger::kIsolateCreated);
 }
 
 
 void Debugger::NotifyCompilation(const Function& func) {
   if (src_breakpoints_ == NULL) {
     // Return with minimal overhead if there are no breakpoints.
@@ skipping 166 matching lines @@
 }
 
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 }  // namespace dart