Clean up promises and fix an edge case bug

test/mjsunit/regress/regress-3641.js

+// Copyright 2015 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
+
+// If a Promise's then method is overridden, that should be respected
+// even if the promise is already resolved. x's resolution function is
+// only called by Promise.resolve(); there shouldn't be a resolution
+// check before when calling x.then.
+
+
+// Async assert framework copied from mjsunit/es6/promises.js
+
+var asyncAssertsExpected = 0;
+
+function assertAsyncRan() { ++asyncAssertsExpected }
+
+function assertLater(f, name) {
+  assertFalse(f()); // should not be true synchronously
+  ++asyncAssertsExpected;
+  var iterations = 0;
+  function runAssertion() {
+    if (f()) {
+      print(name, "succeeded");
+      --asyncAssertsExpected;
+    } else if (iterations++ < 10) {
+      %EnqueueMicrotask(runAssertion);
+    } else {
+      %AbortJS(name + " FAILED!");
+    }
+  }
+  %EnqueueMicrotask(runAssertion);
+}
+
+function assertAsyncDone(iteration) {
+  var iteration = iteration || 0;
+  %EnqueueMicrotask(function() {
+    if (asyncAssertsExpected === 0)
+      assertAsync(true, "all")
+    else if (iteration > 10)  // Shouldn't take more.
+      assertAsync(false, "all... " + asyncAssertsExpected)
+    else
+      assertAsyncDone(iteration + 1)
+  });
+}
+
+// End async assert framework
+
+var y;
+var x = Promise.resolve();
+x.then = () => { y = true; }
+Promise.resolve().then(() => x);
+assertLater(() => y === true, "y === true");
+
+assertAsyncDone();