Added new version of reduce.

sdk/lib/collection/collections.dart

Index: sdk/lib/collection/collections.dart
diff --git a/sdk/lib/collection/collections.dart b/sdk/lib/collection/collections.dart
index f51d98daaabeb87b5ab5bb1613a0f12e4fd5cc6e..95d5a43dedf80fdd0e63f73b14b3e08c63a2b94f 100644
--- a/sdk/lib/collection/collections.dart
+++ b/sdk/lib/collection/collections.dart
@@ -39,9 +39,14 @@ class IterableMixinWorkaround {
   }
 
   static dynamic reduce(Iterable iterable,
-                        dynamic initialValue,
-                        dynamic combine(dynamic previousValue, element)) {
-    return fold(iterable, initialValue, combine);
+                        dynamic combine(previousValue, element)) {
+    Iterator iterator = iterable.iterator;
+    if (!iterator.moveNext()) throw new StateError("No elements");
+    var value = iterator.current;
+    while (iterator.moveNext()) {
+      value = combine(value, iterator.current);
+    }
+    return value;
   }
 
   static dynamic fold(Iterable iterable,