Add v8::Promise::Then.

src/promise.js

Index: src/promise.js
diff --git a/src/promise.js b/src/promise.js
index f7d6307f8be487b0e9c7fd06ae1dd8ae8c2f29f5..710abadbeb8a0517c5e49dde9646b29361d3d843 100644
--- a/src/promise.js
+++ b/src/promise.js
@@ -17,6 +17,7 @@ var PromiseResolve;
 var PromiseReject;
 var PromiseChain;
 var PromiseCatch;
+var PromiseThen;
 
 // mirror-debugger.js currently uses builtins.promiseStatus. It would be nice
 // if we could move these property names into the closure below.
@@ -220,7 +221,7 @@ var promiseRaw = GLOBAL_PRIVATE("Promise#raw");
 
   // Multi-unwrapped chaining with thenable coercion.
 
-  function PromiseThen(onResolve, onReject) {
+  PromiseThen = function PromiseThen(onResolve, onReject) {
     onResolve = IS_SPEC_FUNCTION(onResolve) ? onResolve
                                             : PromiseIdResolveHandler;
     onReject = IS_SPEC_FUNCTION(onReject) ? onReject