refactor: add promise helper and change whenReady to be native impl (#13115)

* Add promise helper and change whenReady to be native impl

* remove commented code

* add GetInner helper to dedupe promise code

* add Promise.reject helper to be consistent with JS

* fix linting

* update promise impl per feedback

* remove param name from unused isolate

* Use non-depreceated resolvers for promises

* Add thread dchecks for promise helper, intiialize promise pointer to nullptr

atom/common/promise_util.cc

@@ -0,0 +1,37 @@
+// Copyright (c) 2018 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#include "atom/common/promise_util.h"
+
+#include <string>
+
+namespace atom {
+
+namespace util {
+
+v8::Maybe<bool> Promise::RejectWithErrorMessage(const std::string& string) {
+  v8::Local<v8::String> error_message =
+      v8::String::NewFromUtf8(isolate(), string.c_str());
+  v8::Local<v8::Value> error = v8::Exception::Error(error_message);
+  return GetInner()->Reject(isolate()->GetCurrentContext(),
+                            mate::ConvertToV8(isolate(), error));
+}
+
+v8::Local<v8::Object> Promise::GetHandle() const {
+  return GetInner()->GetPromise();
+}
+
+}  // namespace util
+
+}  // namespace atom
+
+namespace mate {
+
+v8::Local<v8::Value> mate::Converter<atom::util::Promise*>::ToV8(
+    v8::Isolate*,
+    atom::util::Promise* val) {
+  return val->GetHandle();
+}
+
+}  // namespace mate