Change ListIterator to only check for concurrent modification at each iteration

sdk/lib/_internal/compiler/js_lib/js_array.dart

 // Copyright (c) 2012, 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.
 
 part of _interceptors;
 
 /**
  * The interceptor class for [List]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
@@ skipping 587 matching lines @@
  * 'isGrowable' and 'isMutable' checks into the getInterceptor implementation so
  * these classes can have specialized implementations. Doing so will challenge
  * many assuptions in the JS backend.
  */
 class JSMutableArray<E> extends JSArray<E> implements JSMutableIndexable {}
 class JSFixedArray<E> extends JSMutableArray<E> {}
 class JSExtendableArray<E> extends JSMutableArray<E> {}
 
 
 /// An [Iterator] that iterates a JSArray.
-///
 class ArrayIterator<E> implements Iterator<E> {
   final JSArray<E> _iterable;
-  final int _length;
+  final int _originalLength;
   int _index;
   E _current;
 
   ArrayIterator(JSArray<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;
-
-    // We have to do the length check even on fixed length Arrays.  If we can
-    // inline moveNext() we might be able to GVN the length and eliminate this
-    // check on known fixed length JSArray.
-    if (_length != length) {
+    // 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;
+    }
+    // Check for concurrent modification only at the end in production mode.
+    if (_originalLength != _iterable.length) {
       throw new ConcurrentModificationError(_iterable);
     }
-
-    if (_index >= length) {
-      _current = null;
-      return false;
-    }
-    _current = _iterable[_index];
-    _index++;
-    return true;
+    _current = null;
+    return false;
   }
 }