fkleon/math-expressions — pull request #1 by markhats (2013-12-10 18:30:07+00:00)
Title: Unary minus issue

The unary minus operator seems to not work. A simple input of "-2" causes the following error:

```dart
Breaking on exception: RangeError: -1

Exception: RangeError: -1
Parser.parse (package:math_expressions/src/parser.dart:55:38)
```