Eliminate dependencies on pkg/front_end.

pkg/analyzer_plugin/tool/spec/to_html.dart

 // Copyright (c) 2017, 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.
 
 /**
  * Code for displaying the API as HTML.  This is used both for generating a
  * full description of the API as a web page, and for generating doc comments
  * in generated code.
  */
 import 'dart:convert';
 
 import 'package:analyzer/src/codegen/html.dart';
 import 'package:analyzer/src/codegen/tools.dart';
-import 'package:front_end/src/codegen/tools.dart';
 import 'package:html/dom.dart' as dom;
 
 import 'api.dart';
 import 'from_html.dart';
 
 /**
  * Embedded stylesheet
  */
 final String stylesheet = '''
 body {
@@ skipping 794 matching lines @@
     bool verticalBarNeeded = false;
     for (TypeDecl choice in typeUnion.choices) {
       if (verticalBarNeeded) {
         write(' | ');
       }
       visitTypeDecl(choice);
       verticalBarNeeded = true;
     }
   }
 }