Remove dependency on scala-xml

[allanrenucci]

We still bring in scala-xml to run our tests. We should:

• Report an error when compiling xml literals and scala-xml is not on the classpath similarly to scalac.
• -Ystop-after:parser for tests that parse xml
• And then completely remove the dependency

[smarter]

-Ystop-after:parser for tests that parse xml

but there could be issues related to xml support that only manifest themselves after parsing (e.g. I think #3235 fixed a crash in Typer#assertPositioned)

[allanrenucci]

but there could be issues related to xml support that only manifest themselves after parsing

True. But is seems to be the scalac approach (to be confirmed) so I wouldn't try harder than scalac to have good test coverage for xml literals

e.g. I think #3235 fixed a crash in Typer#assertPositioned

I can only find calls to checkPos from: https://github.com/lampepfl/dotty/blob/7c8dee571d936433925befc56edffd00834654dc/compiler/src/dotty/tools/dotc/typer/FrontEnd.scala#L49

Nothing prevent us to keep checking this after parsing

[smarter]

so I wouldn't try harder than scalac

scalac has the advantage that scala-xml is built using it and has its own test suite, if we add scala-xml to our community build and run its tests then I guess that would be good enough too.

[SethTisue]

scala-xml would probably accept a pull request adding Dotty to their .travis.yml. (IMO this would complement, not replace, adding scala-xml to the Dotty community build.)