[promises] dont create resolving closures in PromiseThen

src/js/promise.js

Index: src/js/promise.js
diff --git a/src/js/promise.js b/src/js/promise.js
index 793d60fb0a7fa29f5f828da7dee849d5d42dabb1..62a218f0106a53f95fc02831a6b87aa0c284e6aa 100644
--- a/src/js/promise.js
+++ b/src/js/promise.js
@@ -169,9 +169,21 @@ function PromiseHandle(value, handler, deferred) {
   try {
     if (debug_is_active) %DebugPushPromise(deferred.promise);
     var result = handler(value);
-    deferred.resolve(result);
+    if (IS_UNDEFINED(deferred.resolve)) {
+      ResolvePromise(deferred.promise, result);
+    } else {
+      deferred.resolve(result);
+    }
   } %catch (exception) {  // Natives syntax to mark this catch block.
-    try { deferred.reject(exception); } catch (e) { }
+    try {
+      if (IS_UNDEFINED(deferred.reject)) {
+        // Pass false for debugEvent so .then chaining does not trigger
+        // redundant ExceptionEvents.
+        RejectPromise(deferred.promise, exception, false);
+      } else {
+        deferred.reject(exception);
+      }
+    } catch (e) { }
   } finally {
     if (debug_is_active) %DebugPopPromise();
   }
@@ -452,9 +464,22 @@ function PromiseThen(onResolve, onReject) {
   }
 
   var constructor = SpeciesConstructor(this, GlobalPromise);
-  // Pass false for debugEvent so .then chaining does not trigger
-  // redundant ExceptionEvents.
-  var resultCapability = NewPromiseCapability(constructor, false);
+  var resultCapability;
+
+  // The resultCapability.promise is only ever fulfilled internally,
+  // so we don't need the closures to protect against accidentally
+  // calling them multiple times.
+  if (constructor === GlobalPromise) {
+    resultCapability = {

[adamk, 2016/10/05 23:05:40]

Can you add a TODO for a followup patch to combine this into
NewPromiseCapability? I don't want to block the 20% improvement on that refactor
though.

+      promise: PromiseCreate(),
+      resolve: UNDEFINED,
+      reject: UNDEFINED
+    };
+  } else {
+    // Pass false for debugEvent so .then chaining does not trigger
+    // redundant ExceptionEvents.
+    resultCapability = NewPromiseCapability(constructor, false);
+  }
   return PerformPromiseThen(this, onResolve, onReject, resultCapability);
 }