[kernel] Move RoundTripVerifier to pkg/kernel/lib/text and rename it

Change-Id: Icb61feab76be161e99b8c2fd316e41271470182a
Reviewed-on: https://dart-review.googlesource.com/c/88560
Reviewed-by: Kevin Millikin <kmillikin@google.com>

pkg/kernel/lib/text/text_serialization_verifier.dart

@@ -51,7 +51,7 @@ class RoundTripMismatchFailure extends RoundTripFailure {
       : super(uri, offset);
 }
 
-class RoundTripVerifier implements Visitor<void> {
+class TextSerializationVerifier implements Visitor<void> {
   /// List of errors produced during round trips on the visited nodes.
   final List<RoundTripFailure> errors = <RoundTripFailure>[];
 
@@ -59,7 +59,7 @@ class RoundTripVerifier implements Visitor<void> {
 
   int lastSeenOffset = noOffset;
 
-  RoundTripVerifier() {
+  TextSerializationVerifier() {
     initializeSerializers();
   }