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

pkg/analyzer/lib/src/dart/element/member.dart

 // Copyright (c) 2014, 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.dart.element.member;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/type.dart';
@@ skipping 424 matching lines @@
    * Return the type in which the element is defined.
    */
   ParameterizedType get definingType => _definingType;
 
   @override
   String get displayName => _baseElement.displayName;
 
   @override
   SourceRange get docRange => _baseElement.docRange;
 
+  @override
+  String get documentationComment => _baseElement.documentationComment;
+
   int get id => _baseElement.id;
 
   @override
   bool get isDeprecated => _baseElement.isDeprecated;
 
   @override
   bool get isOverride => _baseElement.isOverride;
 
   @override
   bool get isPrivate => _baseElement.isPrivate;
@@ skipping 25 matching lines @@
   @override
   int get nameOffset => _baseElement.nameOffset;
 
   @override
   Source get source => _baseElement.source;
 
   @override
   CompilationUnit get unit => _baseElement.unit;
 
   @override
-  String computeDocumentationComment() =>
-      _baseElement.computeDocumentationComment();
+  String computeDocumentationComment() => documentationComment;
 
   @override
   AstNode computeNode() => _baseElement.computeNode();
 
   @override
   Element getAncestor(Predicate<Element> predicate) =>
       baseElement.getAncestor(predicate);
 
   @override
   String getExtendedDisplayName(String shortName) =>
@@ skipping 454 matching lines @@
   bool get isStatic => baseElement.isStatic;
 
   @override
   void visitChildren(ElementVisitor visitor) {
     // TODO(brianwilkerson) We need to finish implementing the accessors used
     // below so that we can safely invoke them.
     super.visitChildren(visitor);
     safelyVisitChild(baseElement.initializer, visitor);
   }
 }