[Navigator] A method subtract to compute the diff between stack mutation.

Summary:
When mutation of a stack happens, we'd like to compute the diff of the stacks (before and after) so that
we can know which routes are removed in the new stack.

This diff adds a new method `substract` which does what we need.

Libraries/CustomComponents/Navigator/Navigation/NavigationRouteStack.js

@@ -11,7 +11,7 @@ var invariant = require('invariant');
 
 type IterationCallback = (route: any, index: number, key: string) => void;
 
-var {List} = immutable;
+var {List, Set} = immutable;
 
 function isRouteEmpty(route: any): boolean {
   return (route === undefined || route === null || route === '') || false;
@@ -32,6 +32,12 @@ class RouteNode {
   }
 }
 
+var StackDiffRecord = immutable.Record({
+  key: null,
+  route: null,
+  index: null,
+});
+
 /**
  * The immutable route stack.
  */
@@ -201,6 +207,25 @@ class RouteStack {
     }
   }
 
+  /**
+   * Returns a Set excluding any routes contained within the stack given.
+   */
+  subtract(stack: RouteStack): Set<StackDiffRecord> {
+    var items = [];
+    this._routeNodes.forEach((node: RouteNode, index: number) => {
+      if (!stack._routeNodes.contains(node)) {
+        items.push(
+          new StackDiffRecord({
+            route: node.value,
+            index: index,
+            key: node.key,
+          })
+        );
+      }
+    });
+    return new Set(items);
+  }
+
   _update(index: number, routeNodes: List): RouteStack {
     if (this._index === index && this._routeNodes === routeNodes) {
       return this;