improve fasta interpolation recovery

pkg/front_end/test/scanner_replacement_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' show UTF8;
 
 import 'package:front_end/src/fasta/scanner/recover.dart'
     show defaultRecoveryStrategy;
 import 'package:front_end/src/fasta/scanner.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/token.dart' as fasta;
@@ skipping 12 matching lines @@
   });
 }
 
 /// 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_Replacement extends ScannerTest {
+class ScannerTest_Replacement extends ScannerTestBase {
+  ScannerTest_Replacement() {
+    usingFasta = true;
+  }
+
   @override
   analyzer.Token scanWithListener(String source, ErrorListener listener,
       {bool genericMethodComments: false,
       bool lazyAssignmentOperators: false}) {
+    // Process the source similar to
+    // pkg/analyzer/lib/src/dart/scanner/scanner.dart
+    // to simulate replacing the analyzer scanner
+
     fasta.ScannerResult result = fasta.scanString(source,
         includeComments: true,
         scanGenericMethodComments: genericMethodComments,
         scanLazyAssignmentOperators: lazyAssignmentOperators,
         recover: ((List<int> bytes, fasta.Token tokens, List<int> lineStarts) {
       // perform recovery as a separate step
       // so that the token stream can be validated before and after recovery
       return tokens;
     }));
+
     fasta.Token tokens = result.tokens;
     assertValidTokenStream(tokens);
     assertValidBeginTokens(tokens);
     if (result.hasErrors) {
       List<int> bytes = UTF8.encode(source);
       tokens = defaultRecoveryStrategy(bytes, tokens, result.lineStarts);
       assertValidTokenStream(tokens, errorsFirst: true);
     }
+
+    // fasta pretends there is an additional line at EOF
+    result.lineStarts.removeLast();
+
     return extractErrors(tokens, listener);
   }
 
   void _assertOpenClosePair(String source) {
     analyzer.BeginToken open = _scan(source);
     fasta.Token close = open.next;
     expect(close.next.isEof, isTrue);
     expect(open.endGroup, close);
     expect(open.isSynthetic, isFalse);
     expect(close.isSynthetic, isFalse);
@@ skipping 287 matching lines @@
       } else if (token is fasta.UnmatchedToken) {
         expect(lastClosedGroup?.endGroup?.next, same(token),
             reason: 'Unexpected error token for group: $lastClosedGroup');
         expect(token.begin, lastClosedGroup);
       }
       token = token.next;
     }
     expect(openerStack, isEmpty, reason: 'Missing closers');
   }
 }