Cache element docs (and add to completions) (#23694).

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index 32590d98d48d3474249f1f491dbbe3ad6a2be2e5..c9604ef38f0e7b42ed6c5f8b7d1ba687e7aefcc7 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -558,22 +558,8 @@ class AnalysisContextImpl implements InternalAnalysisContext {
   }
 
   @override
-  String computeDocumentationComment(Element element) {
-    if (element == null) {
-      return null;
-    }
-    Source source = element.source;
-    if (source == null) {
-      return null;
-    }
-    SourceRange docRange = element.docRange;
-    if (docRange == null) {
-      return null;
-    }
-    String code = getContents(source).data;
-    String comment = code.substring(docRange.offset, docRange.end);
-    return comment.replaceAll('\r\n', '\n');
-  }
+  String computeDocumentationComment(Element element) =>
+      element?.documentationComment;
 
   @override
   List<AnalysisError> computeErrors(Source source) {