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

pkg/front_end/lib/src/fasta/kernel/kernel_field_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.kernel_field_builder;
 
 import 'package:kernel/ast.dart'
     show DartType, Expression, Field, Name, NullLiteral;
 
 import '../../scanner/token.dart' show Token;
@@ skipping 19 matching lines @@
 import 'kernel_shadow_ast.dart' show KernelField;
 
 class KernelFieldBuilder extends FieldBuilder<Expression> {
   final KernelField field;
   final List<MetadataBuilder> metadata;
   final KernelTypeBuilder type;
   final Token initializerTokenForInference;
   final bool hasInitializer;
 
   KernelFieldBuilder(
+      String documentationComment,
       this.metadata,
       this.type,
       String name,
       int modifiers,
       Builder compilationUnit,
       int charOffset,
       this.initializerTokenForInference,
       this.hasInitializer)
       : field = new KernelField(null, fileUri: compilationUnit?.relativeFileUri)
           ..fileOffset = charOffset,
-        super(name, modifiers, compilationUnit, charOffset);
+        super(
+            documentationComment, name, modifiers, compilationUnit, charOffset);
 
   void set initializer(Expression value) {
     if (!hasInitializer && value is! NullLiteral && !isConst && !isFinal) {
       internalProblem(
           messageInternalProblemAlreadyInitialized, charOffset, fileUri);
     }
     field.initializer = value..parent = field;
   }
 
   bool get isEligibleForInference =>
       type == null && (hasInitializer || isInstanceMember);
 
   Field build(SourceLibraryBuilder library) {
+    field.documentationComment = documentationComment;
     field.name ??= new Name(name, library.target);
     if (type != null) {
       field.type = type.build(library);
     }
     bool isInstanceMember = !isStatic && !isTopLevel;
     field
       ..isFinal = isFinal
       ..isConst = isConst
       ..hasImplicitGetter = isInstanceMember
       ..hasImplicitSetter = isInstanceMember && !isConst && !isFinal
@@ skipping 36 matching lines @@
       }
     }
   }
 
   @override
   DartType get builtType => field.type;
 
   @override
   bool get hasImplicitType => type == null;
 }