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

pkg/front_end/lib/src/fasta/builder/procedure_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.procedure_builder;
 
 // Note: we're deliberately using AsyncMarker and ProcedureKind from kernel
 // outside the kernel-specific builders. This is simpler than creating
 // additional enums.
 import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
@@ skipping 16 matching lines @@
 
   final T returnType;
 
   final String name;
 
   final List<TypeVariableBuilder> typeVariables;
 
   final List<FormalParameterBuilder> formals;
 
   ProcedureBuilder(
+      String documentationComment,
       this.metadata,
       this.modifiers,
       this.returnType,
       this.name,
       this.typeVariables,
       this.formals,
       LibraryBuilder compilationUnit,
       int charOffset)
-      : super(compilationUnit, charOffset);
+      : super(compilationUnit, charOffset, documentationComment);
 
   String get debugName => "ProcedureBuilder";
 
   AsyncMarker get asyncModifier;
 
   ProcedureKind get kind;
 
   bool get isConstructor => false;
 
   bool get isRegularMethod => identical(ProcedureKind.Method, kind);
@@ skipping 57 matching lines @@
     return new Scope(local, null, parent, isModifiable: false);
   }
 
   FormalParameterBuilder getFormal(String name) {
     for (FormalParameterBuilder formal in formals) {
       if (formal.name == name) return formal;
     }
     return null;
   }
 }