use lazy attribute initialization instead of initializer lists.

[laszlokorte]

https://github.com/dart-lang/dart-box2d/issues/9

In some cases in which clone(other)-constructors use initializer lists so set the properties I have kept the initializer lists for all constructors.

[dmah42]

Thanks for this patch - it makes the code so much more dart-like and cleaner.

Just a couple of comments in line and I'll merge it.

[laszlokorte]

fixed