Migrate test block 3 to Dart 2.0.

tests/corelib_2/collection_removes_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:collection';
 import "package:expect/expect.dart";
 
 testRemove(base) {
   int length = base.length;
   for (int i = 0; i < length; i++) {
@@ skipping 110 matching lines @@
   int get length => _source.length;
   void set length(int length) {
     _source.length = length;
   }
 
   E operator [](int index) => _source[index];
   void operator []=(int index, E value) {
     _source[index] = value;
   }
 }