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

pkg/analyzer/lib/src/generated/element_handle.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.generated.element_handle;
 
 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/generated/ast.dart';
@@ skipping 321 matching lines @@
   @override
   AnalysisContext get context => _resynthesizer.context;
 
   @override
   String get displayName => actualElement.displayName;
 
   @override
   SourceRange get docRange => actualElement.docRange;
 
   @override
+  String get documentationComment => actualElement.documentationComment;
+
+  @override
   Element get enclosingElement => actualElement.enclosingElement;
 
   @override
   int get hashCode => _location.hashCode;
 
   @override
   bool get isDeprecated => actualElement.isDeprecated;
 
   @override
   bool get isOverride => actualElement.isOverride;
@@ skipping 33 matching lines @@
   CompilationUnit get unit => actualElement.unit;
 
   @override
   bool operator ==(Object object) =>
       object is Element && object.location == _location;
 
   @override
   accept(ElementVisitor visitor) => actualElement.accept(visitor);
 
   @override
-  String computeDocumentationComment() =>
-      actualElement.computeDocumentationComment();
+  String computeDocumentationComment() => documentationComment;
 
   @override
   AstNode computeNode() => actualElement.computeNode();
 
   @override
   Element getAncestor(Predicate<Element> predicate) =>
       actualElement.getAncestor(predicate);
 
   @override
   String getExtendedDisplayName(String shortName) =>
@@ skipping 680 matching lines @@
 }
 
 /**
  * TODO(scheglov) invalid implementation
  */
 class WeakReference<T> {
   final T value;
   WeakReference(this.value);
   T get() => value;
 }