generate synthetic and error tokens when missing digit

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

 // Copyright (c) 2012, 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.
 
 library fasta.scanner.abstract_scanner;
 
 import 'dart:collection' show ListMixin;
 
 import 'dart:typed_data' show Uint16List, Uint32List;
 
@@ skipping 146 matching lines @@
    * substring string [5,9).
    *
    * Note that [extraOffset] can only be used if the covered character(s) are
    * known to be ASCII.
    */
   void appendSubstringToken(TokenType type, int start, bool asciiOnly,
       [int extraOffset]);
 
   /**
    * Appends a substring from the scan offset [start] to the current
-   * [scanOffset] plus [closingQuotes]. The closing quote(s) will be added
+   * [scanOffset] plus [syntheticChars]. The additional char(s) will be added
    * to the unterminated string literal's lexeme but the returned
-   * token's length will *not* include those closing quotes
+   * token's length will *not* include those additional char(s)
    * so as to be true to the original source.
    */
   void appendSyntheticSubstringToken(
-      TokenType type, int start, bool asciiOnly, String closingQuotes);
+      TokenType type, int start, bool asciiOnly, String syntheticChars);
 
   /** Documentation in subclass [ArrayBasedScanner]. */
   void appendPrecedenceToken(TokenType type);
 
   /** Documentation in subclass [ArrayBasedScanner]. */
   int select(int choice, TokenType yes, TokenType no);
 
   /** Documentation in subclass [ArrayBasedScanner]. */
   void appendKeywordToken(Keyword keyword);
 
@@ skipping 535 matching lines @@
         next = advance();
         if (identical(next, $PLUS) || identical(next, $MINUS)) {
           next = advance();
         }
         bool hasExponentDigits = false;
         while (true) {
           if ($0 <= next && next <= $9) {
             hasExponentDigits = true;
           } else {
             if (!hasExponentDigits) {
-              unterminated('1e', shouldAdvance: false);
+              appendSyntheticSubstringToken(TokenType.DOUBLE, start, true, '0');
+              appendErrorToken(
+                  new UnterminatedToken('1e', tokenStart, stringOffset));
               return next;
             }
             break;
           }
           next = advance();
         }
 
         done = true;
         continue LOOP;
       } else {
@@ skipping 579 matching lines @@
       switchToUint32(newLength);
     }
   }
 
   void switchToUint32(int newLength) {
     final newArray = new Uint32List(newLength);
     newArray.setRange(0, arrayLength, array);
     array = newArray;
   }
 }