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

pkg/front_end/lib/src/fasta/source/source_class_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_class_builder.dart b/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
index 62ebbbd1ade789f5dd4081c81a73dc34a821a7d8..e0fc854522e3deb35b171b7f2c667b901fae3480 100644
--- a/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
@@ -49,12 +49,14 @@ Class initializeClass(
 
 class SourceClassBuilder extends KernelClassBuilder {
   final Class cls;
+  final String documentationComment;
 
   final List<ConstructorReferenceBuilder> constructorReferences;
 
   KernelTypeBuilder mixedInType;
 
   SourceClassBuilder(
+      this.documentationComment,
       List<MetadataBuilder> metadata,
       int modifiers,
       String name,
@@ -86,6 +88,8 @@ class SourceClassBuilder extends KernelClassBuilder {
   }
 
   Class build(KernelLibraryBuilder library, LibraryBuilder coreLibrary) {
+    cls.documentationComment = documentationComment;
+
     void buildBuilders(String name, Builder builder) {
       do {
         if (builder is KernelFieldBuilder) {