add fasta.scanner support for lazy assignment operators

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 17 matching lines @@
 /// 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 {
   @override
   analyzer.Token scanWithListener(String source, ErrorListener listener,
       {bool genericMethodComments: false,
       bool lazyAssignmentOperators: false}) {
-    // Note: Fasta always supports lazy assignment operators (`&&=` and `||=`),
-    // so we can ignore the `lazyAssignmentOperators` flag.
-    // TODO(danrubel): once lazyAssignmentOperators are fully supported by
-    // Dart, remove this flag.
     fasta.ScannerResult result = fasta.scanString(source,
-        includeComments: true, scanGenericMethodComments: genericMethodComments,
+        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);
     }
     return extractErrors(tokens, listener);
   }
 
-  @override
-  @failingTest
-  void test_ampersand_ampersand_eq() {
-    // TODO(paulberry,ahe): Fasta scanner doesn't support lazy assignment
-    // operators.
-    super.test_ampersand_ampersand_eq();
-  }
-
-  @override
-  @failingTest
-  void test_bar_bar_eq() {
-    // TODO(paulberry,ahe): Fasta scanner doesn't support lazy assignment
-    // operators.
-    super.test_bar_bar_eq();
-  }
-
   void _assertOpenClosePair(String source) {
     fasta.BeginGroupToken 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);
   }
 
   void _assertOpenOnly(String source) {
@@ skipping 283 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');
   }
 }