Introduce simple Thread class to support gradual refactoring of interfaces.

runtime/vm/base_isolate.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_BASE_ISOLATE_H_
 #define VM_BASE_ISOLATE_H_
 
 namespace dart {
 
 class HandleScope;
 class StackResource;
 class Zone;
 
 // A BaseIsolate contains just enough functionality to allocate
 // StackResources.  This allows us to inline the StackResource
 // constructor/destructor for performance.
 class BaseIsolate {
  public:
   StackResource* top_resource() const { return top_resource_; }
   void set_top_resource(StackResource* value) { top_resource_ = value; }
 
+  // DEPRECATED: Use Thread::current_zone.
   Zone* current_zone() const { return current_zone_; }
   void set_current_zone(Zone* zone) { current_zone_ = zone; }
 
   HandleScope* top_handle_scope() const {
 #if defined(DEBUG)
     return top_handle_scope_;
 #else
     return 0;
 #endif
   }
@@ skipping 91 matching lines @@
 #endif
   int32_t no_callback_scope_depth_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(BaseIsolate);
 };
 
 }  // namespace dart
 
 #endif  // VM_BASE_ISOLATE_H_