Revert "Change ListIterator to only check for concurrent modification at each iteration"

tests/corelib/list_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 "dart:typed_data";
 import "package:expect/expect.dart";
 
 void main() {
   // Typed lists - fixed length and can only contain integers.
@@ skipping 358 matching lines @@
 
   list.replaceRange(2, 3, [5, 5, 5]);
   Expect.listEquals([1, 2, 5, 5, 5, 0, 0, 7, 2, 3, 2, 1], list);
 
   list.replaceRange(2, 4, [6, 6]);
   Expect.listEquals([1, 2, 6, 6, 5, 0, 0, 7, 2, 3, 2, 1], list);
 
   list.replaceRange(6, 8, []);
   Expect.listEquals([1, 2, 6, 6, 5, 0, 2, 3, 2, 1], list);
 
-  // Operations that change the length may cause ConcurrentModificationError.
-  // Reducing the length may cause a RangeError before the
-  // ConcurrentModificationError in production mode.
-  bool checkedMode = false;
-  assert((checkedMode = true));
-
+  // Operations that change the length cause ConcurrentModificationError.
   void testConcurrentModification(action()) {
     testIterator(int when) {
       list.length = 4;
       list.setAll(0, [0, 1, 2, 3]);
       Expect.throws(() {
         for (var element in list) {
-          if (element == when) {
-            when = -1;  // Prevent triggering more than once.
-            action();
-          }
+          if (element == when) action();
         }
-      }, (e) => !checkedMode || e is ConcurrentModificationError);
+      }, (e) => e is ConcurrentModificationError);
     }
     testForEach(int when) {
       list.length = 4;
       list.setAll(0, [0, 1, 2, 3]);
       Expect.throws(() {
         list.forEach((var element) {
-          if (element == when) {
-            when = -1;
-            action();
-          }
+          if (element == when) action();
         });
       }, (e) => e is ConcurrentModificationError);
     }
     // Test the change at different points of the iteration.
     testIterator(0);
     testIterator(1);
     testIterator(3);
     testForEach(0);
     testForEach(1);
     testForEach(3);
@@ skipping 91 matching lines @@
 
 void testListConstructor() {
   Expect.throws(() { new List(0).add(4); });  // Is fixed-length.
   Expect.throws(() { new List(-2); });  // Not negative. /// 01: ok
   Expect.throws(() { new List(null); });  // Not null.
   Expect.listEquals([4], new List()..add(4));
   Expect.throws(() { new List.filled(0, 42).add(4); });  // Is fixed-length.
   Expect.throws(() { new List.filled(-2, 42); });  // Not negative.
   Expect.throws(() { new List.filled(null, 42); });  // Not null.
 }