[promises] Add %WaitForPromise runtime call to allow tests to reliably wait for promises to be fini…

test/mjsunit/basic-promise.js

Index: test/mjsunit/basic-promise.js
diff --git a/test/mjsunit/basic-promise.js b/test/mjsunit/basic-promise.js
new file mode 100644
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--- /dev/null
+++ b/test/mjsunit/basic-promise.js
@@ -0,0 +1,50 @@
+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+// We have to patch mjsunit because normal assertion failures just throw
+// exceptions which are swallowed in a then clause.
+failWithMessage = (msg) => %AbortJS(msg);
+
+let decrement = () => { %DecrementWaitCount(); }
+let increment = () => { %IncrementWaitCount(); }
+
+function WaitForPromise(p) {
+  increment();
+  p.then(decrement, decrement);
+}
+
+function newPromise() {
+  var outerResolve;
+  var outerReject;
+  let promise = new Promise((resolve, reject) => {
+    outerResolve = resolve;
+    outerReject = reject;
+  });
+  WaitForPromise(promise);  // explicitly wait for promise to resolve.
+  return {
+    resolve: outerResolve,
+    reject: outerReject,
+    then: (f, g) => promise.then(f, g)
+  };
+}
+
+(function ResolveOK() {
+  let promise = newPromise();
+  promise.then(msg => {print("resolved: " + msg); assertEquals("ok", msg); },
+               ex => {print("rejected: " + ex); %AbortJS("" + ex); });
+
+  promise.resolve("ok");
+  promise.reject(11); // ignored
+})();
+
+(function RejectOK() {
+  let promise = newPromise();
+  promise.then(msg => {print("resolved: " + msg); %AbortJS("fail"); },
+               ex => {print("rejected: " + ex); assertEquals(42, ex); });
+
+  promise.reject(42);
+  promise.resolve("fail"); // ignored
+})();