Add DartDebugger and move mojo_converter

This CL moves DartDebugger into this library because its dependencies
are cleaner here. Also, move mojo_converter into its own source set so
that folks can link against //lib/tonic without pulling in
//mojo/system.

Change-Id: I6e5e470c4c5702e901f969ec586d2337f294b403
6 files changed