Add a callback to inform embedders when an isolate is being shut down.

vm/isolate.h

Index: vm/isolate.h
===================================================================
--- vm/isolate.h	(revision 7933)
+++ vm/isolate.h	(working copy)
@@ -173,12 +173,27 @@
 
   Debugger* debugger() const { return debugger_; }
 
-  static void SetCreateCallback(Dart_IsolateCreateCallback cback);
-  static Dart_IsolateCreateCallback CreateCallback();
+  static void SetCreateCallback(Dart_IsolateCreateCallback cb) {
+    create_callback_ = cb;
+  }
+  static Dart_IsolateCreateCallback CreateCallback() {
+    return create_callback_;
+  }
 
-  static void SetInterruptCallback(Dart_IsolateInterruptCallback cback);
-  static Dart_IsolateInterruptCallback InterruptCallback();
+  static void SetInterruptCallback(Dart_IsolateInterruptCallback cb) {
+    interrupt_callback_ = cb;
+  }
+  static Dart_IsolateInterruptCallback InterruptCallback() {
+    return interrupt_callback_;
+  }
 
+  static void SetShutdownCallback(Dart_IsolateShutdownCallback cb) {
+    shutdown_callback_ = cb;
+  }
+  static Dart_IsolateShutdownCallback ShutdownCallback() {
+    return shutdown_callback_;
+  }
+
   GcPrologueCallbacks& gc_prologue_callbacks() {
     return gc_prologue_callbacks_;
   }
@@ -229,6 +244,7 @@
 
   static Dart_IsolateCreateCallback create_callback_;
   static Dart_IsolateInterruptCallback interrupt_callback_;
+  static Dart_IsolateShutdownCallback shutdown_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(Isolate);
 };