Implement 'as' operator.

tests/language/cast_test.dart

+// Copyright (c) 2012, 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.
+// Dart test program for constructors and initializers.
+
+// Test 'expression as Type' casts.
+
+class C {
+  final int foo = 42;
+}
+
+class D extends C {
+  final int bar = 37;
+}
+
+Object createC() => new C();
+Object createD() => new D();
+Object getNull() => null;
+Object createList() => <int>[2];
+Object createInt() => 87;
+Object createString() => "a string";
+
+main() {
+  Object oc = createC();
+  Object od = createD();
+  Object on = getNull();
+  Object ol = createList();
+  Object oi = createInt();
+  Object os = createString();
+
+  Expect.equals(42, (oc as C).foo);
+  Expect.equals(42, (od as C).foo);
+  Expect.equals(42, (od as D).foo);
+  Expect.equals(37, (od as D).bar);
+  Expect.equals(37, ((od as C) as D).bar);
+  (oc as D).foo;  /// 01: runtime error
+  (on as D).foo;  /// 02: runtime error
+  (on as C).foo;  /// 03: runtime error
+  oc.foo;  /// 04: static type warning
+  od.foo;  /// 05: static type warning
+  on.foo;  /// 06: runtime error
+  (on as Object).toString();
+  (oc as Object).toString();
+  (od as Object).toString();
+  (on as Dynamic).foo;  /// 07: runtime error
+  (oc as Dynamic).foo;
+  (od as Dynamic).foo;
+  (oc as Dynamic).bar;  /// 08: runtime error
+  (od as Dynamic).bar;
+  C c = oc as C;
+  c = od as C;
+  c = oc;  // 09: static type warning

[ahe, 2012/06/07 13:12:59]

Use three slashes. Some problem elsewhere in this file.

+  D d = od as D;
+  d = oc as D;  /// 10: runtime error
+  d = od;  // 11: static type warning
+
+  (ol as List)[0];
+  (ol as List<int>)[0];
+  (ol as Dynamic)[0];
+  (ol as String).length;  /// 12: runtime error
+  int x = (ol as List<int>)[0];
+  (ol as List<int>)[0] = (oi as int);
+
+  (os as String).length;
+  (os as Dynamic).length;
+  (oi as String).length;  /// 13: runtime error
+  (os as List).length;  /// 14: runtime error
+
+  (oi as int) + 2;
+  (oi as List).length;  /// 15: runtime error
+}
+
+