Added expand method to iterables.

tests/corelib/iterable_expand_test.dart

+// Copyright (c) 2013, 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.
+
+main() {
+  test(expectation, iterable) {
+    Expect.listEquals(expectation, iterable.toList());
+  }
+
+  // Function not called on empty iterable.
+  test([], [].expand((x) { throw "not called"; }));
+
+  // Creating the iterable doesn't call the function.
+  [1].expand((x) { throw "not called"; });
+
+  test([1], [1].expand((x) => [x]));
+  test([1, 2, 3], [1, 2, 3].expand((x) => [x]));
+
+  test([], [1].expand((x) => []));
+  test([], [1, 2, 3].expand((x) => []));
+  test([2], [1, 2, 3].expand((x) => x == 2 ? [2] : []));
+
+  test([1, 1, 2, 2, 3, 3], [1, 2, 3].expand((x) => [x, x]));
+  test([1, 1, 2], [1, 2, 3].expand((x) => [x, x, x].skip(x)));
+
+  // if function throws, iteration is stopped.
+  Iterable iterable = [1, 2, 3].expand((x) {
+    if (x == 2) throw "FAIL";
+    return [x, x];
+  });
+  Iterator it = iterable.iterator;
+  Expect.isTrue(it.moveNext());
+  Expect.equals(1, it.current);
+  Expect.isTrue(it.moveNext());
+  Expect.equals(1, it.current);
+  Expect.throws(it.moveNext, (e) => e == "FAIL");
+  // After throwing, iteration is ended.
+  Expect.equals(null, it.current);
+  Expect.isFalse(it.moveNext());
+}