Fix for 15979- class aliases now have missing concrete members warnings generated on them.

tests/language/mixin_bound_test.dart

 // Copyright (c) 2014, 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.
 
 import "package:expect/expect.dart";
 
 // library abstract_expressions;
 
 abstract class AbstractExpression {}
 
@@ skipping 83 matching lines @@
 }
 
 // library expressions;
 
 // import 'abstractExpressions.dart';
 // import 'evaluator.dart';
 // import 'multiplication.dart';
 // import 'multiplicationEvaluator.dart';
 // import 'stringConverter.dart';
 
-class Expression =
+abstract class Expression =
   AbstractExpression with ExpressionWithEval, ExpressionWithStringConversion;
 
 class Addition =
   AbstractAddition<Expression> with AdditionWithEval<Expression>,
                                     AdditionWithStringConversion<Expression> implements Expression;
 
 class Subtraction =
   AbstractSubtraction<Expression> with SubtractionWithEval<Expression>,
                                        SubtractionWithStringConversion<Expression> implements Expression;
 
 class Number =
   AbstractNumber with NumberWithEval,
                       NumberWithStringConversion implements Expression;
 
 
 class Multiplication =
   AbstractMultiplication<Expression> with MultiplicationWithEval<Expression>,
                                           MultiplicationWithStringConversion<Expression> implements Expression;
 
 
 void main() {
   Expression e = new Multiplication(new Addition(new Number(4), new Number(2)),
                                     new Subtraction(new Number(10), new Number(7)));
   Expect.equals('((4 + 2)) * (10 - 7)) = 18', '$e = ${e.eval}');
 }