Read overlay content when computing completions (issue 27887)

pkg/analysis_server/lib/src/provisional/completion/completion_core.dart

Index: pkg/analysis_server/lib/src/provisional/completion/completion_core.dart
diff --git a/pkg/analysis_server/lib/src/provisional/completion/completion_core.dart b/pkg/analysis_server/lib/src/provisional/completion/completion_core.dart
index 06eb530baa547fc254577fe5b1bde31e82a7b932..b3582c90c7804ff50e763d1b416369b75be4ae3a 100644
--- a/pkg/analysis_server/lib/src/provisional/completion/completion_core.dart
+++ b/pkg/analysis_server/lib/src/provisional/completion/completion_core.dart
@@ -88,6 +88,12 @@ abstract class CompletionRequest {
   Source get source;
 
   /**
+   * Return the content of the [source] in which the completion is being
+   * requested, or `null` if the content could not be accessed.
+   */
+  String get sourceContents;
+
+  /**
    * Throw [AbortCompletion] if the completion request has been aborted.
    */
   void checkAborted();