Update AssistProcessor to use ChangeBuilder

pkg/analyzer_plugin/lib/utilities/change_builder/change_builder_core.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 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
 import 'package:analyzer_plugin/src/utilities/change_builder/change_builder_core.dart';
 import 'package:meta/meta.dart';
@@ skipping 52 matching lines @@
    * as suggestions to the group with the given kind.
    *
    * Throws an [ArgumentError] if either [kind] or [suggestions] are provided
    * without the other.
    */
   void addSimpleLinkedEdit(String groupName, String text,
       {LinkedEditSuggestionKind kind, List<String> suggestions});
 
   /**
    * Set the selection to the given location within the edit being built.
+   *
+   * This method only works correctly if all of the edits that will applied to
+   * text before the current edit have already been created. Those edits are
+   * needed in order to convert the current offset (as of the time this method
+   * is invoked) into an offset relative to the text resulting from applying all
+   * of the edits.
    */
-  @experimental
   void selectHere();
 
   /**
    * Add the given [string] to the content of the current edit.
    */
   void write(String string);
 
   /**
    * Add the given [string] to the content of the current edit and then add an
    * end-of-line marker.
@@ skipping 23 matching lines @@
    *     addReplacement(new SourceRange(offset, 0), buildEdit);
    */
   void addInsertion(int offset, void buildEdit(EditBuilder builder));
 
   /**
    * Add the region of text specified by the given [range] to the linked edit
    * group with the given [groupName]. The [range] is relative to the original
    * source. This is typically used to include pre-existing regions of text in a
    * group. If the region to be included is part of newly generated text, then
    * the method [EditBuilder.addLinkedEdit] should be used instead.
+   *
+   * This method only works correctly if all of the edits that will applied to
+   * text before the given range have already been created. Those edits are
+   * needed in order to convert the range into a range relative to the text
+   * resulting from applying all of the edits.
    */
   void addLinkedPosition(SourceRange range, String groupName);
 
   /**
    * Add a replacement of text specified by the given [range]. The [range] is
    * relative to the original source. The [buildEdit] function is used to write
    * the text that will replace the specified region.
    */
   void addReplacement(SourceRange range, void buildEdit(EditBuilder builder));
 
@@ skipping 39 matching lines @@
    * Add the given [string] to the content of the current edit.
    */
   void write(String string);
 
   /**
    * Add the given [string] to the content of the current edit and then add an
    * end-of-line marker.
    */
   void writeln([String string]);
 }