Add concurrent modification check to ListMixin/IterableMixin.reduce.

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 58301a7ca1cdfb1441491e738b545e073fe7f241..8e0f4b28ff2124b0de5b46c329e123b09d00dfd2 100644
--- a/sdk/lib/internal/iterable.dart
+++ b/sdk/lib/internal/iterable.dart
@@ -175,10 +175,15 @@ abstract class ListIterable<E> extends IterableBase<E>
   Iterable map(f(E element)) => new MappedListIterable(this, f);
 
   E reduce(E combine(var value, E element)) {
+    int length = this.length;
     if (length == 0) throw IterableElementError.noElement();
     E value = elementAt(0);
     for (int i = 1; i < length; i++) {
       value = combine(value, elementAt(i));
+      if (length != this.length) {
+        throw new ConcurrentModificationError(this);
+      }
+
     }
     return value;
   }