Add documentationComment for Class to Kernel. Parse it. Resynthesize in Analyzer.

pkg/kernel/lib/ast.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.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 578 matching lines @@
   /// up, or -1 ([TreeNode.noOffset]) if the file end offset is not available
   /// (this is the default if none is specifically set).
   int fileEndOffset = TreeNode.noOffset;
 
   /// Offset of the declaration, set and used when writing the binary.
   int binaryOffset = -1;
 
   /// The degree to which the contents of the class have been loaded.
   ClassLevel level = ClassLevel.Body;
 
+  /// Documentation comment of the class, or `null`.
+  @informative
+  String documentationComment;
+
   /// List of metadata annotations on the class.
   ///
   /// This defaults to an immutable empty list. Use [addAnnotation] to add
   /// annotations if needed.
   List<Expression> annotations = const <Expression>[];
 
   /// Name of the class.
   ///
   /// Must be non-null and must be unique within the library.
   ///
@@ skipping 4018 matching lines @@
 /// typedef has not been assigned a canonical name yet.
 ///
 /// Returns `null` if the typedef is `null`.
 CanonicalName getCanonicalNameOfTypedef(Typedef typedef_) {
   if (typedef_ == null) return null;
   if (typedef_.canonicalName == null) {
     throw '$typedef_ has no canonical name';
   }
   return typedef_.canonicalName;
 }
+
+/// Annotation describing information which is not part of Dart semantics; in
+/// other words, if this information (or any information it refers to) changes,
+/// static analysis and runtime behavior of the library are unaffected.
+const informative = null;