feat(facade): add ListWrapper.flatten

modules/angular2/src/facade/collection.ts

@@ -278,6 +278,11 @@ export class ListWrapper {
   }
 
   static isImmutable(list: any[]): boolean { return Object.isSealed(list); }
+  static flatten<T>(array: T[][]): T[] {
+    let res = [];
+    array.forEach((a) => res = res.concat(a));
+    return res;
+  }
 }
 
 export function isListLikeIterable(obj: any): boolean {