Morris Fails to Compile with the Closure Compiler

So closure-compiler decided that `long` is a reserved JS keyword, even when it's a property of an object (`this.long`). The fault is on closure-compiler - it's not compliant with the specification - but it means that Morris will fail to build on every asset system that uses the closure compiler.

To fix this I just changed `@long` inside Donut to `@is_long`.