improve handling of missing interpolation identifier

pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart

Index: pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
index 343ea4da7da466ccd0878ffc01ca5736c7cc907a..07b462190fa00be995dac8b3300a9303c186383f 100644
--- a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
@@ -1083,6 +1083,8 @@ abstract class AbstractScanner implements Scanner {
       beginToken(); // The identifier starts here.
       next = tokenizeKeywordOrIdentifier(next, false);
     } else {
+      beginToken(); // The synthetic identifier starts here.
+      appendSyntheticSubstringToken(TokenType.IDENTIFIER, scanOffset, true, '');
       unterminated(r'$', shouldAdvance: false);
     }
     beginToken(); // The string interpolation suffix starts here.