Update fasta scanner tests to refer to #28665.

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 0839db218a3658bc72576e9fb2bd82a55c8c0900..4d9f955876a072a1ce878c831cd07b7c86c16fff 100644
--- a/pkg/front_end/test/scanner_fasta_test.dart
+++ b/pkg/front_end/test/scanner_fasta_test.dart
@@ -168,6 +168,7 @@ class ScannerTest_Fasta extends ScannerTestBase {
   @failingTest
   void test_index() {
     // TODO(paulberry,ahe): "[]" should be parsed as a single token.
+    // See dartbug.com/28665.
     super.test_index();
   }
 
@@ -175,6 +176,7 @@ class ScannerTest_Fasta extends ScannerTestBase {
   @failingTest
   void test_index_eq() {
     // TODO(paulberry,ahe): "[]=" should be parsed as a single token.
+    // See dartbug.com/28665.
     super.test_index_eq();
   }