Fix language tests wrt. warnings.

tests/language/constructor_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.
 // Dart test program for constructors and initializers.
 
 import "package:expect/expect.dart";
 
 class A extends B {
   A(x, y) : super(y), a = x { }
 
   var a;
 }
 
 
 class B {
   var b;
 
   B(x) : b = x { }
 
   B.namedB(var x) : b = x {}
 }
 
 
 // Test the order of initialization: first the instance variable then
 // the super constructor.
-class Alpha {
+abstract class Alpha {
   Alpha(v) {
     this.foo(v);
   }
+  foo(v) => throw 'Alpha.foo should never be called.';
 }
 
 class Beta extends Alpha {
   Beta(v) : super(v), b = 1 {}
 
   foo(v) {
     // Check that 'b' was initialized.
     Expect.equals(1, b);
     b = v;
   }
@@ skipping 13 matching lines @@
     Expect.equals(22, o.a + o.b + o1.b);
 
     var beta = new Beta(3);
     Expect.equals(3, beta.b);
   }
 }
 
 main() {
   ConstructorTest.testMain();
 }