add test for internal scanner API

pkg/front_end/test/scanner_fasta_test.dart

Index: pkg/front_end/test/scanner_fasta_test.dart
diff --git a/pkg/front_end/test/scanner_fasta_test.dart b/pkg/front_end/test/scanner_fasta_test.dart
index 12e12e87a6564e30710c8988ad6d9b778fe72da2..dcf80823bdc1325fa574ffec53e54e81141200fb 100644
--- a/pkg/front_end/test/scanner_fasta_test.dart
+++ b/pkg/front_end/test/scanner_fasta_test.dart
@@ -5,6 +5,7 @@ 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;
@@ -20,6 +21,7 @@ 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);
@@ -27,6 +29,27 @@ main() {
 }
 
 @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}) {