remove old fasta missing exponent recovery

pkg/front_end/lib/src/fasta/scanner/recover.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 licenset hat can be found in the LICENSE file.
 
 library fasta.scanner.recover;
 
 import '../../scanner/token.dart' show TokenType;
 
 import '../fasta_codes.dart'
     show
@@ skipping 110 matching lines @@
     if (append) {
       codeUnits.addAll(next.lexeme.codeUnits);
       next = next.next;
     }
     String value = new String.fromCharCodes(codeUnits);
     return synthesizeToken(charOffset, value, TokenType.IDENTIFIER)
       ..next = next;
   }
 
   recoverExponent() {
-    return synthesizeToken(errorTail.charOffset, "NaN", TokenType.DOUBLE)
-      ..next = errorTail.next;
+    return errorTail.next;
   }
 
   recoverString() {
     return errorTail.next;
   }
 
   recoverHexDigit() {
     return synthesizeToken(errorTail.charOffset, "0", TokenType.INT)
       ..next = errorTail.next;
   }
@@ skipping 95 matching lines @@
 
 String closeBraceFor(String openBrace) {
   return const {
     '(': ')',
     '[': ']',
     '{': '}',
     '<': '>',
     r'${': '}',
   }[openBrace];
 }