Add V8 extra utils for promise state

src/js/prologue.js

Index: src/js/prologue.js
diff --git a/src/js/prologue.js b/src/js/prologue.js
index 59949344656b14a13069ff1f44cb05da9add3eb7..92d99e007214631b6a3938f3e4a9605d97ad50cb 100644
--- a/src/js/prologue.js
+++ b/src/js/prologue.js
@@ -218,6 +218,16 @@ extrasUtils.markPromiseAsHandled = function markPromiseAsHandled(promise) {
   %PromiseMarkAsHandled(promise);
 };
 
+extrasUtils.promiseState = function promiseState(promise) {
+  return %PromiseStatus(promise);

[Dan Ehrenberg, 2017/03/30 12:05:53]

Is it OK for V8 extras to crash when not-a-promise is passed in here, or should
it throw a neat exception?

+};
+
+// [[PromiseState]] values (for extrasUtils.promiseState())
+// These values should be kept in sync with PromiseStatus in globals.h
+extrasUtils.kPROMISE_PENDING = 0;
+extrasUtils.kPROMISE_FULFILLED = 1;
+extrasUtils.kPROMISE_REJECTED = 2;
+
 %ToFastProperties(extrasUtils);
 
 })