Create the JavaScript constructor dynamically.

tests/language/class_codegen_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.
+
+class A {
+  A() : x = 3;
+  foo() => x;
+  var x;
+}
+
+class B extends A {
+  bar() => 499;
+}
+
+class C extends A {
+  bar() => 42;
+}
+
+main() {
+  // We don't instantiate A, but the codegen still needs to emit (parts of) it
+  // for inheritence purposes.
+  var b = new B();
+  var c = new C();
+  Expect.equals(3, b.foo());
+  Expect.equals(3, c.foo());
+  Expect.equals(499, b.bar());
+  Expect.equals(42, c.bar());
+}