Improve the error messages for IndexOutOfRangeException.

corelib/src/exceptions.dart

Index: corelib/src/exceptions.dart
diff --git a/corelib/src/exceptions.dart b/corelib/src/exceptions.dart
index b309b8d6646d4920afe74329330463932a360d3c..60a29af25ff4843207633245660d4e16c4ceedcd 100644
--- a/corelib/src/exceptions.dart
+++ b/corelib/src/exceptions.dart
@@ -16,9 +16,11 @@ interface Exception default ExceptionImplementation {
  * Exception thrown because of an index outside of the valid range.
  */
 class IndexOutOfRangeException implements Exception {
-  const IndexOutOfRangeException(int this._index);
-  String toString() => "IndexOutOfRangeException: $_index";
-  final int _index;
+  const IndexOutOfRangeException(this._value);
+
+  String toString() => "IndexOutOfRangeException: $_value";
+
+  final _value;
 }