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

pkg/front_end/lib/src/fasta/builder/field_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/field_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/field_builder.dart b/pkg/front_end/lib/src/fasta/builder/field_builder.dart
index 7782a3be9ad82491d4cf50aba209ac0931a0247c..bf574d10e6eb633ebe9323209e437dbdb557655f 100644
--- a/pkg/front_end/lib/src/fasta/builder/field_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/field_builder.dart
@@ -13,9 +13,9 @@ abstract class FieldBuilder<T> extends MemberBuilder {
 
   final int modifiers;
 
-  FieldBuilder(
-      this.name, this.modifiers, LibraryBuilder compilationUnit, int charOffset)
-      : super(compilationUnit, charOffset);
+  FieldBuilder(String documentationComment, this.name, this.modifiers,
+      LibraryBuilder compilationUnit, int charOffset)
+      : super(compilationUnit, charOffset, documentationComment);
 
   String get debugName => "FieldBuilder";