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

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index aa87ce4365967d195807defef4037c6bfd6cce83..3cd869b03f61ae03a8232c95f5ca021e020124d3 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -834,6 +834,10 @@ abstract class Member extends NamedNode {
   /// (this is the default if none is specifically set).
   int fileEndOffset = TreeNode.noOffset;
 
+  /// Documentation comment of the member, or `null`.
+  @informative
+  String documentationComment;
+
   /// List of metadata annotations on the member.
   ///
   /// This defaults to an immutable empty list. Use [addAnnotation] to add