Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/js/nodes.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 NodeVisitor<T> {
+abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
   T visitExpressionStatement(ExpressionStatement node);
   T visitEmptyStatement(EmptyStatement node);
   T visitIf(If node);
   T visitFor(For node);
   T visitForIn(ForIn node);
   T visitWhile(While node);
   T visitDo(Do node);
@@ skipping 814 matching lines @@
   /** Contains the pattern and the flags.*/
   String pattern;
 
   RegExpLiteral(this.pattern);
 
   accept(NodeVisitor visitor) => visitor.visitRegExpLiteral(this);
   void visitChildren(NodeVisitor visitor) {}
 
   int get precedenceLevel => PRIMARY;
 }