src: remove global HandleScope

Make it more difficult to accidentally leak handles by removing the
top-level HandleScope.  Now if there's no valid HandleScope now, V8
will complain and, in debug builds, abort.

src/node.cc

@@ -3249,8 +3249,11 @@ int Start(int argc, char** argv) {
     Locker locker(node_isolate);
     Environment* env =
         CreateEnvironment(node_isolate, argc, argv, exec_argc, exec_argv);
+    // This Context::Scope is here so EnableDebug() can look up the current
+    // environment with Environment::GetCurrentChecked().
+    // TODO(bnoordhuis) Reorder the debugger initialization logic so it can
+    // be removed.
     Context::Scope context_scope(env->context());
-    HandleScope handle_scope(env->isolate());
     uv_run(env->event_loop(), UV_RUN_DEFAULT);
     EmitExit(env);
     RunAtExit(env);

src/tls_wrap.cc

@@ -235,9 +235,8 @@ void TLSCallbacks::SSLInfoCallback(const SSL* ssl_, int where, int ret) {
   SSL* ssl = const_cast<SSL*>(ssl_);
   TLSCallbacks* c = static_cast<TLSCallbacks*>(SSL_get_app_data(ssl));
   Environment* env = c->env();
-  // There should be a Context::Scope a few stack frames down.
-  assert(env->context() == env->isolate()->GetCurrentContext());
   HandleScope handle_scope(env->isolate());
+  Context::Scope context_scope(env->context());
   Local<Object> object = c->object();
 
   if (where & SSL_CB_HANDSHAKE_START) {