Add reduce to Collection.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 9980d5da6ff4edbf18a0b52a7de3afc0ead7cf5e..15d3219910bbda047ca532167fd348aac57f197b 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -88,6 +88,11 @@ class ObjectArray<E> implements List<E> {
     return Collections.map(this, new GrowableObjectArray.withCapacity(length), f);
   }
 
+  Dynamic reduce(Dynamic initialValue,
+                 Dynamic combine(Dynamic previousValue, E element)) {
+    return Collections.reduce(this, initialValue, combine);
+  }
+
   Collection<E> filter(bool f(E element)) {
     return Collections.filter(this, new GrowableObjectArray<E>(), f);
   }
@@ -220,6 +225,11 @@ class ImmutableArray<E> implements List<E> {
     return Collections.map(this, new GrowableObjectArray.withCapacity(length), f);
   }
 
+  Dynamic reduce(Dynamic initialValue,
+                 Dynamic combine(Dynamic previousValue, E element)) {
+    return Collections.reduce(this, initialValue, combine);
+  }
+
   Collection<E> filter(bool f(E element)) {
     return Collections.filter(this, new GrowableObjectArray<E>(), f);
   }