Add reduce to Collection.

tests/corelib/collection_test.dart

+// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class CollectionTest {
+  CollectionTest(Collection collection) {
+    testReduce(collection);
+  }
+
+  void testReduce(Collection collection) {
+    Expect.equals(28, collection.reduce(0, (prev, element) => prev + element));
+    Expect.equals(
+        3024, collection.reduce(1, (prev, element) => prev * element));
+  }
+}
+
+
+main() {
+  final TEST_ELEMENTS = const [4, 2, 6, 7, 9];
+  // Const list.
+  new CollectionTest(TEST_ELEMENTS);
+
+  // Fixed size list.
+  var fixedList = new List(TEST_ELEMENTS.length);
+  for (int i = 0; i < TEST_ELEMENTS.length; i++) {
+    fixedList[i] = TEST_ELEMENTS[i];
+  }
+  new CollectionTest(fixedList);
+
+  // Dynamic size list.
+  new CollectionTest(new List.from(TEST_ELEMENTS));
+
+  // Dynamic size set.
+  new CollectionTest(new Set.from(TEST_ELEMENTS));
+}