[cfe] Add support for classes to round-trip text serialization

Supplementary changes for handling Classes done in this CL are the
following:

  * handling Supertypes
  * handling Constructors
  * handling remaining kinds of Procedure
  * serializing AsyncMarker the same way as flags
  * eliminating the need to wrap flags into IntLiteral

Change-Id: Id69c6e37f093cf80206f4657b649f79c75484775
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154000
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Dmitry Stefantsov <dmitryas@google.com>

pkg/kernel/lib/text/serializer_combinators.dart

@@ -230,7 +230,7 @@ class UriSerializer extends TextSerializer<Uri> {
 // They require a function mapping serializables to a tag string.  This is
 // implemented by Tagger visitors.
 // A tagged union of serializer/deserializers.
-class Case<T extends Node> extends TextSerializer<T> {
+class Case<T> extends TextSerializer<T> {
   final Tagger<T> tagger;
   final List<String> _tags;
   final List<TextSerializer<T>> _serializers;

pkg/kernel/lib/text/text_serialization_verifier.dart

@@ -325,19 +325,13 @@ class VerificationState {
   static bool isSupported(Node node) => !isNotSupported(node);
 
   static bool isNotSupported(Node node) =>
-      node is FunctionNode && node.body == null ||
-      node is Procedure &&
-          (!node.isStatic || node.kind != ProcedureKind.Method) ||
-      node is Class ||
       node is Component ||
-      node is Constructor ||
       node is Extension ||
       node is Library ||
       node is LibraryPart ||
       node is MapEntry ||
       node is Name && node.isPrivate ||
       node is RedirectingFactoryConstructor ||
-      node is Supertype ||
       node is Typedef;
 }
 
@@ -485,6 +479,10 @@ class TextSerializationVerifier extends RecursiveVisitor<void> {
       makeRoundTrip<SwitchCase>(node, switchCaseSerializer);
     } else if (node is Initializer) {
       makeRoundTrip<Initializer>(node, initializerSerializer);
+    } else if (node is Supertype) {
+      makeRoundTrip<Supertype>(node, supertypeSerializer);
+    } else if (node is Class) {
+      makeRoundTrip<Class>(node, classSerializer);
     } else {
       throw new StateError(
           "Don't know how to make a round trip for a supported node "