Update AssistProcessor to use ChangeBuilder

pkg/analyzer_plugin/lib/utilities/change_builder/change_builder_core.dart

Index: pkg/analyzer_plugin/lib/utilities/change_builder/change_builder_core.dart
diff --git a/pkg/analyzer_plugin/lib/utilities/change_builder/change_builder_core.dart b/pkg/analyzer_plugin/lib/utilities/change_builder/change_builder_core.dart
index d6894b10adc538393e7f79b76a7b5b0a1a07f220..c34dfd003ada0e1a09d434c2a0e076cb00c70523 100644
--- a/pkg/analyzer_plugin/lib/utilities/change_builder/change_builder_core.dart
+++ b/pkg/analyzer_plugin/lib/utilities/change_builder/change_builder_core.dart
@@ -70,8 +70,13 @@ abstract class EditBuilder {
 
   /**
    * 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();
 
   /**
@@ -115,6 +120,11 @@ abstract class FileEditBuilder {
    * 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);