Update flow/Set.js definition

Summary: Sync the `Set` definition with the latest flow. https://github.com/facebook/flow/blob/879aa351/lib/core.js#L605-L618

Reviewed By: yungsters

Differential Revision: D4960310

fbshipit-source-id: f7546197e9ebeadbbcc21fa17ad40735659d27df

flow/Set.js

@@ -20,6 +20,7 @@ declare module "Set" {
   // messages about "Using Set instead of Set".
   declare class SetPolyfill<T> {
     @@iterator(): Iterator<T>;
+    constructor(iterable: ?Iterable<T>): void;
     add(value: T): SetPolyfill<T>;
     clear(): void;
     delete(value: T): boolean;