Minor optimizations based on IRHydra2.

The comparison to PROMISE_IDENTITY/PROMISE_THROWER were being handled
as unoptimized calls to a generic strict comparison function; not worth it,
considering these cases are very rarely taken.

Try to avoid polymorphic functions by ensuring that the
PROMISE_FAKE_CAPABILITY is also a valid PromiseCapability.

`x == null` seems to trigger deoptimization at least once; just use
strict equality tests.

lib/promise.js

@@ -69,15 +69,13 @@ function tryCatch1n(Fn, arg1) {
 // for moar speed.
 
 function TypeIsObject(x) {
-  /* jshint eqnull:true */
   // This is expensive when it returns false; use this function
   // when you expect it to return true in the common case.
-  return x != null && Object(x) === x;
+  return x !== void 0 && x !== null && Object(x) === x;
 }
 function TypeIsNotObject(val) {
   // This is more appropriate when TypeIsObject may return false.
-  /* jshint eqnull:true */
-  return val == null || val === true || val === false ||
+  return val === void 0 || val === null || val === true || val === false ||
     typeof val === 'string' || typeof val === 'number';
 }
 
@@ -192,11 +190,7 @@ var fakeRetvalFromThen = false;
 // Constants for Promise implementation
 var PROMISE_IDENTITY = (function PROMISE_IDENTITY(v) { return v; });
 var PROMISE_THROWER = (function PROMISE_THROWER(t) { throw t; });
-var PROMISE_FAKE_CAPABILITY = {
-  resolve: function PROMISE_FAKE_CAPABILITY_RESOLVE() {},
-  reject: function PROMISE_FAKE_CAPABILITY_REJECT() {},
-  promise: { fakePromise: true },
-};
+var PROMISE_FAKE_CAPABILITY = new PromiseCapability(void 0, void 0);
 var PROMISE_PENDING = 0;
 var PROMISE_RESOLVING = 1; // PROMISE_PENDING combined with alreadyResolved
 var PROMISE_FULFILLED = 2;
@@ -237,21 +231,12 @@ function promiseReactionReject(promiseCapability, handlerResult) {
 }
 function promiseReactionJob(handler, promiseCapability, argument) {
   var handlerResult;
-  if (handler === PROMISE_IDENTITY) {
-    if (promiseCapability === PROMISE_FAKE_CAPABILITY) { return; }
-    handlerResult = argument;
-  } else if (handler === PROMISE_THROWER) {
-    if (promiseCapability === PROMISE_FAKE_CAPABILITY) { return; }
-    handlerResult = argument;
+  // Encapsulate try/catch here to avoid deoptimization.
+  handlerResult = tryCatch1(handler, argument);
+  if (promiseCapability === PROMISE_FAKE_CAPABILITY) { return; }
+  if (handlerResult === errorObj) {
+    handlerResult = handlerResult.e;
     return promiseReactionReject(promiseCapability, handlerResult);
-  } else {
-    // Encapsulate try/catch here to avoid deoptimization.
-    handlerResult = tryCatch1(handler, argument);
-    if (promiseCapability === PROMISE_FAKE_CAPABILITY) { return; }
-    if (handlerResult === errorObj) {
-      handlerResult = handlerResult.e;
-      return promiseReactionReject(promiseCapability, handlerResult);
-    }
   }
   return promiseReactionResolve(promiseCapability, handlerResult);
 }