Fix some warnings.

lib/utf/utf_core.dart

Index: lib/utf/utf_core.dart
diff --git a/lib/utf/utf_core.dart b/lib/utf/utf_core.dart
index 01c537594f78190f326548785f83f9e87a164743..7649da7086fa4516409de86fb2e0b39eaba426cc 100644
--- a/lib/utf/utf_core.dart
+++ b/lib/utf/utf_core.dart
@@ -219,13 +219,13 @@ class _ListRange implements Iterable {
       this._offset = offset,
       this._length = (length == null ? source.length - offset : length) {
     if (_offset < 0 || _offset > _source.length) {
-      throw new IndexOutOfRangeException("offset out of range (< 0)");
+      throw new IndexOutOfRangeException(_offset);
     }
     if (_length != null && (_length < 0)) {
-      throw new IndexOutOfRangeException("length out of range (< 0)");
+      throw new IndexOutOfRangeException(_length);
     }
     if (_length + _offset > _source.length) {
-      throw new IndexOutOfRangeException("offset + length > source.length");
+      throw new IndexOutOfRangeException(_length + _offset);
     }
   }