feat(facade): add ListWrapper.flatten

modules/angular2/src/facade/collection.dart

@@ -232,6 +232,14 @@ class ListWrapper {
   static bool isImmutable(List l) {
     return l is UnmodifiableListView;
   }
+
+  static List flatten(List l) {
+    final res = [];
+    l.forEach((item) {
+      res.addAll(item);
+    });
+    return res;
+  }
 }
 
 bool isListLikeIterable(obj) => obj is Iterable;

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 {