use AssertionError for assertions
Fix bug with line breaks after a line comment

If there is, for example, an opening brace ('{') after a line comment,
the intersection of the allowed line breaks after the line comment
(1...) and the allowed line breaks before the other token (0..0) is
empty, which would crash the formatter.

Found in ceylon/ceylon.ast@ee6b7e2
(source/ceylon/ast/redhat/Invocation.ceylon).
Add test for bug with line breaks after line comment

See previous commit for explanation.
CONTRIBUTING.md: Fix GitHub links

lucaswerkmeister -> ceylon
Merge branch 'master' of https://github.com/ceylon/ceylon.formatter