1. Create a port when a debugger object is created for an isolate, use this

vm/debugger.h

 // 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.
 
 #ifndef VM_DEBUGGER_H_
 #define VM_DEBUGGER_H_
 
 #include "include/dart_debugger_api.h"
 
 #include "vm/object.h"
+#include "vm/port.h"
 
 namespace dart {
 
 class SourceBreakpoint;
 class CodeBreakpoint;
 class Isolate;
 class ObjectPointerVisitor;
 class ActiveVariables;
 class RemoteObjectCache;
 
@@ skipping 194 matching lines @@
     kIsolateCreated = 4,
     kIsolateShutdown = 5,
     kIsolateInterrupted = 6,
   };
   struct DebuggerEvent {
     EventType type;
     union {
       DebuggerStackTrace* stack_trace;
       SourceBreakpoint* breakpoint;
       const Object* exception;
-      Isolate* isolate;
+      Dart_Port isolate_id;
     };
   };
   typedef void EventHandler(DebuggerEvent *event);
 
   Debugger();
   ~Debugger();
 
   void Initialize(Isolate* isolate);
   void Shutdown();
   bool IsActive();
@@ skipping 28 matching lines @@
 
   RawArray* GetInstanceFields(const Instance& obj);
   RawArray* GetStaticFields(const Class& cls);
   RawArray* GetLibraryFields(const Library& lib);
   RawArray* GetGlobalFields(const Library& lib);
 
   intptr_t CacheObject(const Object& obj);
   RawObject* GetCachedObject(intptr_t obj_id);
   bool IsValidObjectId(intptr_t obj_id);
 
+  Dart_Port GetIsolateId() { return isolate_id_; }
+
   static void SetEventHandler(EventHandler* handler);
   static void SetBreakpointHandler(BreakpointHandler* handler);
 
   // Utility functions.
   static const char* QualifiedFunctionName(const Function& func);
 
   RawObject* GetInstanceField(const Class& cls,
                               const String& field_name,
                               const Instance& object);
   RawObject* GetStaticField(const Class& cls,
@@ skipping 40 matching lines @@
 
   bool ShouldPauseOnException(DebuggerStackTrace* stack_trace,
                               const Object& exc);
 
   void CollectLibraryFields(const GrowableObjectArray& field_list,
                             const Library& lib,
                             const String& prefix,
                             bool include_private_fields);
 
   Isolate* isolate_;
+  Dart_Port isolate_id_;  // A unique ID for the isolate in the debugger.
   bool initialized_;
 
   // ID number generator.
   intptr_t next_id_;
 
   // Current stack trace. Valid while executing breakpoint callback code.
   DebuggerStackTrace* stack_trace_;
 
   RemoteObjectCache* obj_cache_;
 
@@ skipping 18 matching lines @@
   static EventHandler* event_handler_;
 
   friend class SourceBreakpoint;
   DISALLOW_COPY_AND_ASSIGN(Debugger);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_DEBUGGER_H_