Change toAnalyzerTokenStream into a class.

pkg/front_end/test/scanner_fasta_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/fasta/scanner/error_token.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/keyword.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/string_scanner.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/token.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/token_constants.dart' as fasta;
@@ skipping 23 matching lines @@
       // TODO(paulberry): once the analyzer toolchain no longer needs generic
       // method comments, remove tests that exercise them.
       fail('No generic method comment support in Fasta');
     }
     // Note: Fasta always supports lazy assignment operators (`&&=` and `||=`),
     // so we can ignore the `lazyAssignmentOperators` flag.
     // TODO(paulberry): once lazyAssignmentOperators are fully supported by
     // Dart, remove this flag.
     var scanner = new fasta.StringScanner(source, includeComments: true);
     var token = scanner.tokenize();
-    return toAnalyzerTokenStream(token,
-        (ScannerErrorCode errorCode, int offset, List<Object> arguments) {
-      listener.errors.add(new TestError(offset, errorCode, arguments));
-    });
+    return new ToAnalyzerTokenStreamConverter_WithListener(listener)
+        .convertTokens(token);
   }
 
   @override
   @failingTest
   void test_ampersand_ampersand_eq() {
     // TODO(paulberry,ahe): Fasta doesn't support `&&=` yet
     super.test_ampersand_ampersand_eq();
   }
 
   @override
@@ skipping 310 matching lines @@
 
 /// Scanner tests that exercise the Fasta scanner, then convert the tokens to
 /// analyzer tokens, then convert back to Fasta tokens before checking
 /// assertions.
 @reflectiveTest
 class ScannerTest_Fasta_Roundtrip extends ScannerTest_Fasta_Base {
   @override
   fasta.Token scan(String source) {
     var scanner = new fasta.StringScanner(source, includeComments: true);
     var fastaTokenStream = scanner.tokenize();
-    var analyzerTokenStream = toAnalyzerTokenStream(fastaTokenStream,
-        (ScannerErrorCode errorCode, int offset, List<Object> arguments) {
-      fail('Unexpected error: $errorCode, $offset, $arguments');
-    });
+    var analyzerTokenStream = new ToAnalyzerTokenStreamConverter_NoErrors()
+        .convertTokens(fastaTokenStream);
     return fromAnalyzerTokenStream(analyzerTokenStream);
   }
 }
+
+/// Override of [ToAnalyzerTokenStreamConverter] that verifies that there are no
+/// errors.
+class ToAnalyzerTokenStreamConverter_NoErrors
+    extends ToAnalyzerTokenStreamConverter {
+  @override
+  void reportError(
+      ScannerErrorCode errorCode, int offset, List<Object> arguments) {
+    fail('Unexpected error: $errorCode, $offset, $arguments');
+  }
+}
+
+/// Override of [ToAnalyzerTokenStreamConverter] that records errors in an
+/// [ErrorListener].
+class ToAnalyzerTokenStreamConverter_WithListener
+    extends ToAnalyzerTokenStreamConverter {
+  final ErrorListener _listener;
+
+  ToAnalyzerTokenStreamConverter_WithListener(this._listener);
+
+  @override
+  void reportError(
+      ScannerErrorCode errorCode, int offset, List<Object> arguments) {
+    _listener.errors.add(new TestError(offset, errorCode, arguments));
+  }
+}