dart2js: depend directly on kernel/ast to avoid indirect dependency on dart:io

pkg/compiler/lib/src/kernel/kernel_debug.dart

 // Copyright (c) 2016, 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.
 
 /// Helper for debug Kernel nodes.
 
 library kernel.debug;
 
-import 'package:kernel/kernel.dart';
+import 'package:kernel/ast.dart';
 import 'package:kernel/visitor.dart';
 
 import '../util/util.dart' show Indentation, Tagging;
 
 class DebugPrinter extends Visitor with Indentation, Tagging<Node> {
   StringBuffer sb = new StringBuffer();
 
   void visitNodeWithChildren(Node node, String type, [Map params]) {
     openNode(node, type, params);
     node.visitChildren(this);
@@ skipping 47 matching lines @@
     closeNode();
   }
 
   /// Pretty-prints given node tree into string.
   static String prettyPrint(Node node) {
     var p = new DebugPrinter();
     node.accept(p);
     return p.sb.toString();
   }
 }