Re-add top-level v8::Locker

11d1eca9 added a v8 locker to ease development of 3rd party threading
extensions but it created a condition which would cause node to exit
uncleanly while in debug mode; it was reverted in 7543c38d.

The problem here is that the Locker was being disposed after V8 was torn
down. Adding some scoping fixes that.

src/node.cc

@@ -2644,32 +2644,38 @@ int Start(int argc, char *argv[]) {
   // This needs to run *before* V8::Initialize()
   argv = Init(argc, argv);
 
-  v8::V8::Initialize();
-  v8::HandleScope handle_scope;
-
-  // Create the one and only Context.
-  Persistent<v8::Context> context = v8::Context::New();
-  v8::Context::Scope context_scope(context);
-
-  Handle<Object> process_l = SetupProcessObject(argc, argv);
-  v8_typed_array::AttachBindings(context->Global());
-
-  // Create all the objects, load modules, do everything.
-  // so your next reading stop should be node::Load()!
-  Load(process_l);
-
-  // All our arguments are loaded. We've evaluated all of the scripts. We
-  // might even have created TCP servers. Now we enter the main eventloop. If
-  // there are no watchers on the loop (except for the ones that were
-  // uv_unref'd) then this function exits. As long as there are active
-  // watchers, it blocks.
-  uv_run(uv_default_loop());
-
-  EmitExit(process_l);
+  V8::Initialize();
+  Persistent<Context> context;
+  {
+    Locker locker;
+    HandleScope handle_scope;
+
+    // Create the one and only Context.
+    Persistent<Context> context = Context::New();
+    Context::Scope context_scope(context);
+
+    Handle<Object> process_l = SetupProcessObject(argc, argv);
+    v8_typed_array::AttachBindings(context->Global());
+
+    // Create all the objects, load modules, do everything.
+    // so your next reading stop should be node::Load()!
+    Load(process_l);
+
+    // All our arguments are loaded. We've evaluated all of the scripts. We
+    // might even have created TCP servers. Now we enter the main eventloop. If
+    // there are no watchers on the loop (except for the ones that were
+    // uv_unref'd) then this function exits. As long as there are active
+    // watchers, it blocks.
+    uv_run(uv_default_loop());
+
+    EmitExit(process_l);
+#ifndef NDEBUG
+    context.Dispose();
+#endif
+  }
 
 #ifndef NDEBUG
   // Clean up.
-  context.Dispose();
   V8::Dispose();
 #endif  // NDEBUG