Add Iterable.fold (and Stream.fold) which replace `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 16290e57949335947138e6c74c2225a9702754a8..8729452b7b2cf3b830c0a386a27f23d91529f7f0 100644
--- a/sdk/lib/collection/collections.dart
+++ b/sdk/lib/collection/collections.dart
@@ -40,6 +40,12 @@ class IterableMixinWorkaround {
   static dynamic reduce(Iterable iterable,
                         dynamic initialValue,
                         dynamic combine(dynamic previousValue, element)) {
+    return fold(iterable, initialValue, combine);
+  }
+
+  static dynamic fold(Iterable iterable,
+                      dynamic initialValue,
+                      dynamic combine(dynamic previousValue, element)) {
     for (final element in iterable) {
       initialValue = combine(initialValue, element);
     }