Eliminate dependencies on pkg/front_end.

pkg/analyzer_plugin/lib/utilities/completion/suggestion_builder.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.
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart' hide Element;
 import 'package:analyzer_plugin/utilities/completion/relevance.dart';
-import 'package:front_end/src/base/source.dart' show Source;
+import 'package:analyzer/src/generated/source.dart' show Source;
 
 /**
  * An object used to build code completion suggestions for Dart code.
  */
 abstract class SuggestionBuilder {
   /**
    * Return a suggestion based on the given [element], or `null` if a suggestion
    * is not appropriate for the given element. If the suggestion is not
    * currently in scope, then specify [importForSource] as the source to which
    * an import should be added.
    */
   CompletionSuggestion forElement(Element element,
       {String completion,
       CompletionSuggestionKind kind: CompletionSuggestionKind.INVOCATION,
       int relevance: DART_RELEVANCE_DEFAULT,
       Source importForSource});
 }