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

pkg/kernel/lib/ast.dart

 // Copyright (c) 2016, 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.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 816 matching lines @@
 // ------------------------------------------------------------------------
 //                            MEMBERS
 // ------------------------------------------------------------------------
 
 abstract class Member extends NamedNode {
   /// End offset in the source file it comes from. Valid values are from 0 and
   /// up, or -1 ([TreeNode.noOffset]) if the file end offset is not available
   /// (this is the default if none is specifically set).
   int fileEndOffset = TreeNode.noOffset;
 
+  /// Documentation comment of the member, or `null`.
+  @informative
+  String documentationComment;
+
   /// List of metadata annotations on the member.
   ///
   /// This defaults to an immutable empty list. Use [addAnnotation] to add
   /// annotations if needed.
   List<Expression> annotations = const <Expression>[];
   Name name;
 
   /// Flags summarizing the kinds of AST nodes contained in this member, for
   /// speeding up transformations that only affect certain types of nodes.
   ///
@@ skipping 3829 matching lines @@
   if (typedef_.canonicalName == null) {
     throw '$typedef_ has no canonical name';
   }
   return typedef_.canonicalName;
 }
 
 /// Annotation describing information which is not part of Dart semantics; in
 /// other words, if this information (or any information it refers to) changes,
 /// static analysis and runtime behavior of the library are unaffected.
 const informative = null;