Add extra information to FormatException.

runtime/lib/double_patch.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.
 // Dart core library.
 
 // VM implementation of double.
 
 patch class double {
 
   static double _nativeParse(String str,
                              int start, int end) native "Double_parse";
 
   static double _parse(var str) {
     int len = str.length;
     int start = str._firstNonWhitespace();
     if (start == len) return null;  // All whitespace.
     int end = str._lastNonWhitespace() + 1;
     assert(start < end);
 
     return _nativeParse(str, start, end);
   }
 
   /* patch */ static double parse(String str,
                                   [double onError(String str)]) {
     var result = _parse(str);
     if (result == null) {
-      if (onError == null) throw new FormatException(str);
+      if (onError == null) throw new FormatException("Invalid double", str);
       return onError(str);
     }
     return result;
   }
 }