fix(ListWrapper): make list slice in dart return empty list if start and end are inverted like JS

modules/angular2/src/core/facade/collection.dart

@@ -182,6 +182,10 @@ class ListWrapper {
   }
 
   static List slice(List l, [int from = 0, int to]) {
+    //in JS if from > to an empty array is returned
+    if(to != null && from > to) {
+      return [];
+    }
     return l.sublist(_startOffset(l, from), _endOffset(l, to));
   }
 

modules/angular2/test/core/facade/collection_spec.ts

@@ -62,6 +62,9 @@ export function main() {
 
       it('should support negative end',
          () => { expect(ListWrapper.slice(l, -3, -1)).toEqual([2, 3]); });
+
+      it('should return empty list if start is greater than end',
+         () => { expect(ListWrapper.slice(l, 4, 2)).toEqual([]); });
     });
 
     describe('indexOf', () => {