Stop making classes implicitly abstract

tests/language/abstract_factory_constructor_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.
 
 // Exercises issue 2282, factory constructors in abstract classes should
 // not emit a static type warning
 
 class B extends A1 {
   B() {}
   method() {}
 }
 
 class A1 {
   A1() {}
   abstract method();
   factory A1.make() { return new B(); }
 }
 
 class A2 {
   abstract method();
   A2.make() {}
 }
 
 main() {
   new A1.make();
-  new A2.make();              /// 00: static type warning, runtime error
+  new A2.make();              /// 00: static type warning
 }