Change NotImplementedException to UnimplementedError.

lib/core/exceptions.dart

Index: lib/core/exceptions.dart
diff --git a/lib/core/exceptions.dart b/lib/core/exceptions.dart
index e49856777bd90dcd3e925d94fe220e77c9cf6e45..2d2bc436dd496be9903aa7ac5529310df31ee8d4 100644
--- a/lib/core/exceptions.dart
+++ b/lib/core/exceptions.dart
@@ -61,15 +61,6 @@ class NullPointerException implements Exception {
 }
 
 
-class NotImplementedException implements Exception {
-  const NotImplementedException([String message]) : this._message = message;
-  String toString() => (this._message !== null
-                        ? "NotImplementedException: $_message"
-                        : "NotImplementedException");
-  final String _message;
-}
-
-
 class IllegalJSRegExpException implements Exception {
   const IllegalJSRegExpException(String this._pattern, String this._errmsg);
   String toString() => "IllegalJSRegExpException: '$_pattern' '$_errmsg'";