Use abstract classes instead of interfaces in dart2js.

sdk/lib/_internal/compiler/implementation/constant_system.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.
 
 part of dart2js;
 
-interface Operation {
-  final SourceString name;
+abstract class Operation {
+  SourceString get name;
   bool isUserDefinable();
 }
 
-interface UnaryOperation extends Operation {
+abstract class UnaryOperation extends Operation {
   /** Returns [:null:] if it was unable to fold the operation. */
   Constant fold(Constant constant);
   apply(value);
 }
 
-interface BinaryOperation extends Operation {
+abstract class BinaryOperation extends Operation {
   /** Returns [:null:] if it was unable to fold the operation. */
   Constant fold(Constant left, Constant right);
   apply(left, right);
 }
 
 /**
  * A [ConstantSystem] is responsible for creating constants and folding them.
  */
-interface ConstantSystem {
+abstract class ConstantSystem {
   BinaryOperation get add;
   BinaryOperation get bitAnd;
   UnaryOperation get bitNot;
   BinaryOperation get bitOr;
   BinaryOperation get bitXor;
   BinaryOperation get booleanAnd;
   BinaryOperation get booleanOr;
   BinaryOperation get divide;
   BinaryOperation get equal;
   BinaryOperation get greaterEqual;
@@ skipping 25 matching lines @@
   bool isInt(Constant constant);
   /** Returns true if the [constant] is a double at runtime. */
   bool isDouble(Constant constant);
   /** Returns true if the [constant] is a string at runtime. */
   bool isString(Constant constant);
   /** Returns true if the [constant] is a boolean at runtime. */
   bool isBool(Constant constant);
   /** Returns true if the [constant] is null at runtime. */
   bool isNull(Constant constant);
 }