Start new design for interceptors and implement String.charCodeAt with it.

tests/language/interceptor_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.
+
+// Test that interceptors (that is, methods in classes implemented as
+// JavaScript primitives) in dart2js work.
+
+class A {
+  charCodeAt(a) => a;
+}
+
+main() {
+  var res = [[], 1, 'foo', new A()];
+  Expect.throws(() => res[0].charCodeAt(1));
+  Expect.throws(() => (res[0].charCodeAt)(1));
+
+  Expect.throws(() => res[1].charCodeAt(1));
+  Expect.throws(() => (res[1].charCodeAt)(1));
+
+  Expect.equals(111, res[2].charCodeAt(1));
+  Expect.equals(111, (res[2].charCodeAt)(1));
+  Expect.throws(() => res[2].charCodeAt(1, 4));
+  Expect.throws(() => res[2].charCodeAt());
+  Expect.throws(() => (res[2].charCodeAt)(1, 4));
+  Expect.throws(() => (res[2].charCodeAt)());
+
+  Expect.equals(1, res[3].charCodeAt(1));
+  Expect.equals(1, (res[3].charCodeAt)(1));
+  Expect.throws(() => res[3].charCodeAt(1, 4));
+  Expect.throws(() => res[3].charCodeAt());
+  Expect.throws(() => (res[3].charCodeAt)(1, 4));
+  Expect.throws(() => (res[3].charCodeAt)());
+}