Fix remaining prefer_single_quotes and fail travis on lints (dart-lang/source_span#48)

Missed saving some files in my editor in https://github.com/dart-lang/source_span/pull/47

The travis config was not set to fail for lints.
8 files changed
tree: d5b0e7abe2fc4df1e986ddcd611705a10791aa67
  1. pkgs/