This adds a --no-unicode option to disable Unicode span rendering,
decouples repl highlighting from SourceSpan.highlight, and updates
tests to work with the new error highlighting.
It also tightly scopes source spans for statements with children.
Previously, source spans for these nodes extended all the way through
any whitespace that followed the node. This led to messy-looking
multiline span highlights with dart-lang/source_span#25.
Now, StylesheetParser.children doesn't consume trailing whitespace.
Instead, we add a helper method StylesheetParser._withChildren that
parses children, creates the appropriate span, and then consumes the
trailing whitespace.
