Change toAnalyzerTokenStream into a class.

pkg/front_end/test/scanner_roundtrip_test.dart

 // Copyright (c) 2017, 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.
 
 import 'package:front_end/src/fasta/analyzer/token_utils.dart';
-import 'package:front_end/src/scanner/errors.dart';
 import 'package:front_end/src/scanner/token.dart';
-import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
+import 'scanner_fasta_test.dart';
 import 'scanner_test.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ScannerTest_RoundTrip);
   });
 }
 
 /// Scanner tests that use the analyzer scanner, then convert the resulting
 /// token stream into a Fasta token stream, then convert back to an analyzer
 /// token stream before verifying assertions.
 ///
 /// These tests help to validate the correctness of the analyzer->Fasta token
 /// stream conversion.
 @reflectiveTest
 class ScannerTest_RoundTrip extends ScannerTest {
   @override
   Token scanWithListener(String source, ErrorListener listener,
       {bool genericMethodComments: false,
       bool lazyAssignmentOperators: false}) {
     var analyzerToken = super.scanWithListener(source, listener,
         genericMethodComments: genericMethodComments,
         lazyAssignmentOperators: lazyAssignmentOperators);
     var fastaToken = fromAnalyzerTokenStream(analyzerToken);
-    return toAnalyzerTokenStream(fastaToken,
-        (ScannerErrorCode errorCode, int offset, List<Object> arguments) {
-      // Since [scanWithListener] reports errors to the listener, we don't
-      // expect any error tokens in the Fasta token stream; so this callback
-      // should never be called.
-      fail('Unexpected error reported: $errorCode, $offset, $arguments');
-    });
+    // Since [scanWithListener] reports errors to the listener, we don't
+    // expect any error tokens in the Fasta token stream, so we convert using
+    // ToAnalyzerTokenStreamConverter_NoErrors.
+    return new ToAnalyzerTokenStreamConverter_NoErrors()
+        .convertTokens(fastaToken);
   }
 
   @override
   @failingTest
   void test_ampersand_ampersand_eq() {
     // TODO(paulberry,ahe): Fasta scanner doesn't support lazy assignment
     // operators.
     super.test_ampersand_ampersand_eq();
   }
 
@@ skipping 35 matching lines @@
     super.test_scriptTag_withoutSpace();
   }
 
   @override
   @failingTest
   void test_scriptTag_withSpace() {
     // TODO(paulberry,ahe): Fasta scanner doesn't support script tag
     super.test_scriptTag_withSpace();
   }
 }