Add Iterable.fold (and Stream.fold) which replace `reduce`.

runtime/bin/vmstats/bargraph.dart

Index: runtime/bin/vmstats/bargraph.dart
diff --git a/runtime/bin/vmstats/bargraph.dart b/runtime/bin/vmstats/bargraph.dart
index 502ff8e0b1528d60845d649225ec77cef844a5e6..0b93c8523156f92fb2d459c43087c0d6a92366c8 100644
--- a/runtime/bin/vmstats/bargraph.dart
+++ b/runtime/bin/vmstats/bargraph.dart
@@ -218,6 +218,6 @@ class Sample {
   Iterator<int> get iterator => _segments.iterator;
 
   int total() {
-    return _segments.reduce(0, (int prev, int element) => prev + element);
+    return _segments.fold(0, (int prev, int element) => prev + element);
   }
 }