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

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

 // Copyright (c) 2015, 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.
 
 library analyzer.src.context.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 540 matching lines @@
     }
     _onSourcesChangedController.add(new SourcesChangedEvent(changeSet));
     return new ApplyChangesStatus(changeSet.addedSources.isNotEmpty ||
         changeSet.changedContents.isNotEmpty ||
         changeSet.deletedSources.isNotEmpty ||
         changedSources.isNotEmpty ||
         removedSources.isNotEmpty);
   }
 
   @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) {
     String name = source.shortName;
     if (AnalysisEngine.isHtmlFileName(name)) {
       return computeResult(source, HTML_ERRORS);
     }
     return computeResult(source, DART_ERRORS);
   }
 
@@ skipping 1431 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new IllegalArgumentException(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }