remove old fasta missing exponent recovery

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

Index: pkg/front_end/lib/src/fasta/scanner/recover.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/recover.dart b/pkg/front_end/lib/src/fasta/scanner/recover.dart
index 3f272928ec963e7ec791758da9764c4f02228000..aa9b91c51d2e3dfc01790a4efcbdac5fee65c549 100644
--- a/pkg/front_end/lib/src/fasta/scanner/recover.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/recover.dart
@@ -128,8 +128,7 @@ Token defaultRecoveryStrategy(
   }
 
   recoverExponent() {
-    return synthesizeToken(errorTail.charOffset, "NaN", TokenType.DOUBLE)
-      ..next = errorTail.next;
+    return errorTail.next;
   }
 
   recoverString() {