src: make node_isolate global

V8 3.15 has new API functions that let you specify the Isolate. V8 and
node.js generally spend 0.5-3.5% of the time in pthread_getspecific(),
looking up the current Isolate. Avoid that overhead by making "our"
isolate global so we can pass it around. The change to the new API is
introduced in follow-up commits.

src/node.cc

@@ -148,6 +148,12 @@ static bool use_sni = false;
 // process-relative uptime base, initialized at start-up
 static double prog_start_time;
 
+static volatile bool debugger_running = false;
+static uv_async_t dispatch_debug_messages_async;
+
+// Declared in node_internals.h
+Isolate* node_isolate = NULL;
+
 
 static void Tick(void) {
   // Avoid entering a V8 scope.
@@ -2539,13 +2545,6 @@ static void ParseArgs(int argc, char **argv) {
 }
 
 
-static Isolate* node_isolate = NULL;
-static volatile bool debugger_running = false;
-
-
-static uv_async_t dispatch_debug_messages_async;
-
-
 // Called from the main thread.
 static void DispatchDebugMessagesAsyncCallback(uv_async_t* handle, int status) {
   v8::Debug::ProcessDebugMessages();

src/node_internals.h

@@ -28,6 +28,9 @@
 
 namespace node {
 
+// Defined in node.cc
+extern v8::Isolate* node_isolate;
+
 #ifdef _WIN32
 // emulate snprintf() on windows, _snprintf() doesn't zero-terminate the buffer
 // on overflow...