Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/constants.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.
 
-interface ConstantVisitor<R> {
+abstract class ConstantVisitor<R> {
   R visitSentinel(SentinelConstant constant);
   R visitFunction(FunctionConstant constant);
   R visitNull(NullConstant constant);
   R visitInt(IntConstant constant);
   R visitDouble(DoubleConstant constant);
   R visitTrue(TrueConstant constant);
   R visitFalse(FalseConstant constant);
   R visitString(StringConstant constant);
   R visitList(ListConstant constant);
   R visitMap(MapConstant constant);
@@ skipping 418 matching lines @@
       if (fields[i] != other.fields[i]) return false;
     }
     return true;
   }
 
   int hashCode() => _hashCode;
   List<Constant> getDependencies() => fields;
 
   accept(ConstantVisitor visitor) => visitor.visitConstructed(this);
 }