isolates: remove references to uv_thread_self()

uv_thread_self() is difficult to implement on Windows in a way where the return
value is valid across threads. It's already been removed from upstream libuv.

src/node.cc

@@ -2679,13 +2679,10 @@ int Start(int argc, char *argv[]) {
   v8::V8::Initialize();
   v8::HandleScope handle_scope;
 
-  // Get the id of the this, the main, thread.
-  uv_thread_t tid = uv_thread_self();
-
   // Create the main node::Isolate object
   node::Isolate::Initialize();
   Isolate* isolate = new node::Isolate();
-  isolate->tid_ = tid;
+  isolate->tid_ = (uv_thread_t) -1;
   isolate->Enter();
   StartThread(isolate, argc, argv);
   isolate->Dispose();

src/node_isolate.cc

@@ -219,7 +219,6 @@ void Isolate::OnMessage(IsolateMessage* msg, void* arg) {
   HandleScope scope;
 
   Isolate* self = static_cast<Isolate*>(arg);
-  assert(uv_thread_self() == self->tid_);
   NODE_ISOLATE_CHECK(self);
 
   Buffer* buf = Buffer::New(msg->data_, msg->size_, IsolateMessage::Free, msg);
@@ -426,7 +425,6 @@ struct IsolateWrap: public ObjectWrap {
   }
 
   void OnMessage(IsolateMessage* msg) {
-    assert(uv_thread_self() != isolate_->tid_);
     NODE_ISOLATE_CHECK(parent_isolate_);
     HandleScope scope;
     Buffer* buf = Buffer::New(