Change ListIterator to only check for concurrent modification at each iteration

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

Index: sdk/lib/_internal/compiler/js_lib/js_array.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_array.dart b/sdk/lib/_internal/compiler/js_lib/js_array.dart
index 5763242e399206f4ecaa95aec1d96a3499233df7..5ef836bab35087f075963d3a4f54a4e3a469dd1e 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_array.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_array.dart
@@ -605,34 +605,31 @@ 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;
   }
 }