Add Member.documentationComment and use it to resynthesize documentation from Kernel.

pkg/analyzer/lib/src/dart/element/element.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.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 3947 matching lines @@
       return _kernel.name.name;
     }
     if (serializedExecutable != null) {
       return serializedExecutable.name;
     }
     return super.displayName;
   }
 
   @override
   String get documentationComment {
+    if (_kernel != null) {
+      return _kernel.documentationComment;
+    }
     if (serializedExecutable != null) {
       return serializedExecutable?.documentationComment?.text;
     }
     return super.documentationComment;
   }
 
   /**
    * Set whether this executable element is external.
    */
   void set external(bool isExternal) {
@@ skipping 3571 matching lines @@
   @override
   int get codeOffset {
     if (_unlinkedVariable != null) {
       return _unlinkedVariable.codeRange?.offset;
     }
     return super.codeOffset;
   }
 
   @override
   String get documentationComment {
+    if (_kernel != null) {
+      return _kernel.documentationComment;
+    }
     if (_unlinkedVariable != null) {
       return _unlinkedVariable?.documentationComment?.text;
     }
     return super.documentationComment;
   }
 
   @override
   bool get hasImplicitType {
     if (_unlinkedVariable != null) {
       return _unlinkedVariable.type == null;
@@ skipping 2114 matching lines @@
 
   @override
   DartObject computeConstantValue() => null;
 
   @override
   void visitChildren(ElementVisitor visitor) {
     super.visitChildren(visitor);
     _initializer?.accept(visitor);
   }
 }