Reapply 'Add reduce to Collection.'

corelib/src/implementation/collections.dart

Index: corelib/src/implementation/collections.dart
diff --git a/corelib/src/implementation/collections.dart b/corelib/src/implementation/collections.dart
index 462d33af2a703382ecf34f320b5b8d8ca1a0913a..7786d3d364f1457d4361282fb304d47ad4a1b4d3 100644
--- a/corelib/src/implementation/collections.dart
+++ b/corelib/src/implementation/collections.dart
@@ -35,6 +35,15 @@ class Collections {
     return destination;
   }
 
+  static Dynamic reduce(Iterable iterable,
+                        Dynamic initialValue,
+                        Dynamic combine(Dynamic previousValue, element)) {
+    for (final element in iterable) {
+      initialValue = combine(initialValue, element);
+    }
+    return initialValue;
+  }
+
   static List filter(Iterable source, List destination, bool f(o)) {
     for (final e in source) {
       if (f(e)) destination.add(e);