Added implementation of String.codeUnits.

tests/corelib/string_codeunits_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.
+
+main() {
+  test(String s) {
+    Iterable<int> units = s.codeUnits;
+    List<int> expectedUnits = <int>[];
+    for (int i = 0; i < s.length; i++) {
+      expectedUnits.add(s.codeUnitAt(i));
+    }
+
+    Expect.equals(s.length, units.length);
+    for (int i = 0; i < s.length; i++) {
+      Expect.equals(s.codeUnitAt(i), units.elementAt(i));
+    }
+
+    // for-in
+    var res = [];
+    for (int unit in units) {
+      res.add(unit);
+    }
+    Expect.listEquals(expectedUnits, res);
+
+    // .map
+    Expect.listEquals(expectedUnits.map((x) => x.toRadixString(16)).toList(),
+                      units.map((x) => x.toRadixString(16)).toList());
+  }
+
+  test("abc");
+  test("\x00\u0000\u{000000}");
+  test("\u{ffff}\u{10000}\u{10ffff}");
+  String string = new String.fromCharCodes(
+      [0xdc00, 0xd800, 61, 0xd800, 0xdc00, 62, 0xdc00, 0xd800]);
+  test(string);
+
+  // Setting position in the middle of a surrogate pair is not allowed.
+  var r = new CodeUnits("\u{10000}");
+  var it = r.iterator;
+  Expect.isTrue(it.moveNext());
+  Expect.equals(0xD800, it.current);
+  Expect.isTrue(it.moveNext());
+  Expect.equals(0xDC00, it.current);
+}