adding version constraint to pub spec for breaking sdk changes in dart p...

...arsers

`ChromeDownloads` has fully generated code now. When dart parsers gets fixed will place the version constraint on the package back to `any` and keep the `environment` constraint. 

https://code.google.com/p/parsers/source/detail?r=cef7b645ae0bd68f69708c8e0ec646e4b64b5b5f# is what broke on the parsing side. @devoncarew do you want to remove any addtional try/cath that was added to protect the unit tests? Will unit tests fail if one file does not parse correctly?