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

pkg/front_end/lib/src/fasta/builder/member_builder.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.
 
 library fasta.member_builder;
 
 import 'builder.dart'
     show Builder, ClassBuilder, LibraryBuilder, ModifierBuilder;
 
 abstract class MemberBuilder extends ModifierBuilder {
   /// For top-level members, the parent is set correctly during
   /// construction. However, for class members, the parent is initially the
   /// library and updated later.
   Builder parent;
 
+  String documentationComment;
+
   String get name;
 
-  MemberBuilder(Builder parent, int charOffset)
+  MemberBuilder(Builder parent, int charOffset, this.documentationComment)
       : parent = parent,
         super(parent, charOffset);
 
   bool get isInstanceMember => isClassMember && !isStatic;
 
   bool get isClassMember => parent is ClassBuilder;
 
   bool get isTopLevel => !isClassMember;
 
   bool get isNative => false;
 
   bool get isRedirectingGenerativeConstructor => false;
 
   LibraryBuilder get library {
     if (parent is LibraryBuilder) {
       LibraryBuilder library = parent;
       return library.partOfLibrary ?? library;
     } else {
       ClassBuilder cls = parent;
       return cls.library;
     }
   }
 
   @override
   String get fullNameForErrors => name;
 }