Revert "improve fasta unterminated string recovery"

pkg/front_end/lib/src/fasta/quote.dart

 // Copyright (c) 2016, 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.quote;
 
 import 'errors.dart' show inputError, internalError;
 
 import 'scanner/characters.dart'
     show
@@ skipping 112 matching lines @@
   return unescape(first.substring(firstQuoteLength(first, quote)), quote);
 }
 
 String unescapeLastStringPart(String last, Quote quote) {
   return unescape(
       last.substring(0, last.length - lastQuoteLength(quote)), quote);
 }
 
 String unescapeString(String string) {
   Quote quote = analyzeQuote(string);
-  int startIndex = firstQuoteLength(string, quote);
-  if (startIndex == string.length) {
-    return '';
-  }
-  int endIndex = string.length - lastQuoteLength(quote);
-  return unescape(string.substring(startIndex, endIndex), quote);
+  return unescape(
+      string.substring(firstQuoteLength(string, quote),
+          string.length - lastQuoteLength(quote)),
+      quote);
 }
 
 String unescape(String string, Quote quote) {
   switch (quote) {
     case Quote.Single:
     case Quote.Double:
       return !string.contains("\\")
           ? string
           : unescapeCodeUnits(string.codeUnits, false);
 
@@ skipping 98 matching lines @@
         }
       } else {
         // Nothing, escaped character is passed through;
       }
       if (code > 0x10FFFF) return error(i, invalidCodePoint);
     }
     result[resultOffset++] = code;
   }
   return new String.fromCharCodes(result, 0, resultOffset);
 }