Span.getLocationMessage returns incorrect output for lines without trailing newlines

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#19665

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If a span points to the final line in a file, and that line doesn't end in a newline, the arrows in getLocationMessage will be rendered on the same line as the source text. For example:

    Error on line 12, column 28 of pubspec.yaml: Error loading transformer: I hate these settings!
      - app/src/html_rewriter: {} ^^