Stop making classes implicitly abstract

tests/language/implicit_this_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.
 
 interface Interface {
   final x;
 }
 
-// The abstract keyword is implicit if a class declares an abstract
-// member. This class does not declare any abstract members, so it
-// must be marked abstract because it doesn't implement the x getter.
 abstract class Abstract implements Interface {
   String toString() => x.toString();
 }
 
 // This class does not implement "x" either, but it is not marked
 // abstract.
 class SubAbstract1 extends Abstract { } /// 01: static type warning
 
 // This class is implicitly abstract as it declares an abstract getter
 // method.
@@ skipping 10 matching lines @@
 }
 
 class SubConcrete extends Concrete {
   final x;
   SubConcrete(this.x);
 }
 
 void main() {
   var x = new Abstract(); /// 02: runtime error
   var y = new SubAbstract1(); /// 01: continued
-  var z = new SubAbstract2(); /// 03: runtime error
+  var z = new SubAbstract2();
   var a = new SubSubAbstract2(); /// 04: continued
   Expect.equals(x, x); /// 02: continued
   Expect.equals('7', new Concrete().toString());
   Expect.equals('42', new SubConcrete(42).toString());
   Expect.equals('7', new SubConcrete(new Concrete()).toString());
 }