Change ListIterator to only check for concurrent modification at each iteration

sdk/lib/internal/iterable.dart

Index: sdk/lib/internal/iterable.dart
diff --git a/sdk/lib/internal/iterable.dart b/sdk/lib/internal/iterable.dart
index 47c94f8b0be8975c60745e36a5f9440922feae5b..e783ea092acc7b88dfe2ff8bcc960a82e70d0ae9 100644
--- a/sdk/lib/internal/iterable.dart
+++ b/sdk/lib/internal/iterable.dart
@@ -320,27 +320,30 @@ class SubListIterable<E> extends ListIterable<E> {
  */
 class ListIterator<E> implements Iterator<E> {
   final Iterable<E> _iterable;
-  final int _length;
+  final int _originalLength;
   int _index;
   E _current;
 
   ListIterator(Iterable<E> iterable)
-      : _iterable = iterable, _length = iterable.length, _index = 0;
+      : _iterable = iterable, _originalLength = iterable.length, _index = 0;
 
   E get current => _current;
 
   bool moveNext() {
-    int length = _iterable.length;
-    if (_length != length) {
-      throw new ConcurrentModificationError(_iterable);
+    // Check for concurrent modifiction at each step in checked mode.
+    assert((_originalLength == _iterable.length) ||
+           (throw new ConcurrentModificationError(_iterable)));
+    if (_index < _iterable.length) {
+      _current = _iterable.elementAt(_index);
+      _index++;
+      return true;
     }
-    if (_index >= length) {
-      _current = null;
-      return false;
+    // Check for concurrent modification only at the end in production mode.
+    if (_originalLength != _iterable.length) {
+      throw new ConcurrentModificationError(_iterable);
     }
-    _current = _iterable.elementAt(_index);
-    _index++;
-    return true;
+    _current = null;
+    return false;
   }
 }