add test for internal scanner API

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 'dart:convert';
 
 import 'package:analyzer/src/fasta/token_utils.dart';
 import 'package:front_end/src/fasta/fasta_codes.dart';
+import 'package:front_end/src/fasta/scanner.dart' as usedForFuzzTesting;
 import 'package:front_end/src/fasta/scanner/error_token.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;
 import 'package:front_end/src/fasta/scanner/utf8_bytes_scanner.dart' as fasta;
 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_test.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ScannerTest_Fasta);
+    defineReflectiveTests(ScannerTest_Fasta_FuzzTestAPI);
     defineReflectiveTests(ScannerTest_Fasta_UTF8);
     defineReflectiveTests(ScannerTest_Fasta_Direct);
     defineReflectiveTests(ScannerTest_Fasta_Direct_UTF8);
   });
 }
 
 @reflectiveTest
+class ScannerTest_Fasta_FuzzTestAPI {
+  test_API() {
+    // These two API are used when fuzz testing the scanner.
+    String source = 'class A { }';
+
+    usedForFuzzTesting.ScannerResult result =
+        usedForFuzzTesting.scanString(source);
+    expect(result?.hasErrors, isFalse);
+    expect(result.tokens?.type, same(Keyword.CLASS));
+
+    // UTF8 encode source with trailing zero
+    List<int> bytes = UTF8.encode(source).toList();
+    bytes.add(0);
+
+    result = usedForFuzzTesting.scan(bytes);
+    expect(result?.hasErrors, isFalse);
+    expect(result.tokens?.type, same(Keyword.CLASS));
+  }
+}
+
+@reflectiveTest
 class ScannerTest_Fasta_UTF8 extends ScannerTest_Fasta {
   @override
   createScanner(String source, {bool genericMethodComments: false}) {
     List<int> encoded = UTF8.encode(source).toList(growable: true);
     encoded.add(0); // Ensure 0 terminted bytes for UTF8 scanner
     return new fasta.Utf8BytesScanner(encoded,
         includeComments: true,
         scanGenericMethodComments: genericMethodComments);
   }
 }
@@ skipping 642 matching lines @@
   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));
   }
 }