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

pkg/front_end/lib/src/fasta/dill/dill_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.dill_member_builder;
 
 import 'package:kernel/ast.dart'
     show Constructor, Field, Member, Procedure, ProcedureKind;
 
 import '../kernel/kernel_builder.dart'
     show
         Builder,
         MemberBuilder,
         isRedirectingGenerativeConstructorImplementation;
 
 import '../modifier.dart'
     show abstractMask, constMask, externalMask, finalMask, staticMask;
 
 import '../problems.dart' show unhandled;
 
 class DillMemberBuilder extends MemberBuilder {
   final int modifiers;
 
   final Member member;
 
   DillMemberBuilder(Member member, Builder parent)
       : modifiers = computeModifiers(member),
         member = member,
-        super(parent, member.fileOffset);
+        super(parent, member.fileOffset, member.documentationComment);
 
   String get debugName => "DillMemberBuilder";
 
   Member get target => member;
 
   String get name => member.name.name;
 
   bool get isConstructor => member is Constructor;
 
   ProcedureKind get kind {
@@ skipping 36 matching lines @@
     modifier |= member.isStatic ? staticMask : 0;
   } else if (member is Constructor) {
     modifier |= member.isConst ? constMask : 0;
   } else {
     dynamic parent = member.parent;
     unhandled("${member.runtimeType}", "computeModifiers", member.fileOffset,
         Uri.base.resolve(parent.fileUri));
   }
   return modifier;
 }