Update typings for Promise.all() for D5827319

Reviewed By: samwgoldman

Differential Revision: D5830645

fbshipit-source-id: db2bb0715aab58d3601712ee0b080abaa9f8f131

flow/Promise.js

@@ -29,7 +29,7 @@ declare class Promise<+R> {
   static resolve<T>(object?: Promise<T> | T): Promise<T>;
   static reject<T>(error?: any): Promise<T>;
 
-  static all<Elem, T:Iterable<Elem>>(promises: T): Promise<$TupleMap<T, typeof $await>>;
+  static all<T: Iterable<mixed>>(promises: T): Promise<$TupleMap<T, typeof $await>>;
   static race<T>(promises: Array<Promise<T>>): Promise<T>;
 
   // Non-standard APIs