Add extra information to FormatException.

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 15 matching lines @@
   void handleNumber(num value) {}
   void handleBool(bool value) {}
   void handleNull() {}
   void beginObject() {}
   void propertyName() {}
   void propertyValue() {}
   void endObject() {}
   void beginArray() {}
   void arrayElement() {}
   void endArray() {}
-  /** Called on failure to parse [source]. */
-  void fail(String source, int position, String message) {}
 }
 
 /**
  * A [JsonListener] that builds data objects from the parser events.
  *
  * This is a simple stack-based object builder. It keeps the most recently
  * seen value in a variable, and uses it depending on the following event.
  */
 class _BuildJsonListener extends _JsonListener {
   /**
@@ skipping 513 matching lines @@
     // This correctly creates -0.0 for doubles.
     listener.handleNumber(_parseDouble(source, start, position));
     return position;
   }
 
   static double _parseDouble(String source, int start, int end)
       native "Double_parse";
 
   void fail(int position, [String message]) {
     if (message == null) message = "Unexpected character";
-    listener.fail(source, position, message);
-    // If the listener didn't throw, do it here.
-    String slice;
-    int sliceEnd = position + 20;
-    if (sliceEnd > source.length) {
-      slice = "'${source.substring(position)}'";
-    } else {
-      slice = "'${source.substring(position, sliceEnd)}...'";
-    }
-    throw new FormatException("Unexpected character at $position: $slice");
+    throw new FormatException(message, source, position);
   }
 }
 
 // UTF-8 conversion.
 
 patch class _Utf8Encoder {
   /* patch */ static List<int> _createBuffer(int size) => new Uint8List(size);
 }