Fix error message for unexpected characters in JSON.

runtime/lib/convert_patch.dart

 // Copyright (c) 2013, 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.
 
 import "dart:typed_data";
 
 // JSON conversion.
 
 patch _parseJson(String json, reviver(var key, var value)) {
   _BuildJsonListener listener;
@@ skipping 486 matching lines @@
     // At that time, position points to the next character, and isDouble
     // is set if the literal contains a decimal point or an exponential.
     parsing: {
       if (char == MINUS) {
         intSign = -1;
         position++;
         if (position == length) fail(position, "Missing expected digit");
         char = source.codeUnitAt(position);
       }
       if (char < CHAR_0 || char > CHAR_9) {
-        if (negative) {
+        if (intSign < 0) {
           fail(position, "Missing expected digit");
         } else {
           // If it doesn't even start out as a numeral.
           fail(position, "Unexpected character");
         }
       }
       if (char == CHAR_0) {
         position++;
         if (position == length) break parsing;
         char = source.codeUnitAt(position);
@@ skipping 56 matching lines @@
     if (message == null) message = "Unexpected character";
     throw new FormatException(message, source, position);
   }
 }
 
 // UTF-8 conversion.
 
 patch class _Utf8Encoder {
   /* patch */ static List<int> _createBuffer(int size) => new Uint8List(size);
 }