use span.getLocationMessage in errors logged by pub

[sigmundch]

Span has support for highlighting errors, since TransformLogger already takes this information, we should use it when printing error messages in the console.

For example, here is how the polymer-linter would normally print to the console:

 var useColors = stdioType(stdout) == StdioType.TERMINAL;
  var levelColor = (kind == 'error') ? _RED_COLOR : _MAGENTA_COLOR;
  var output = new StringBuffer();
  if (useColors) output.write(levelColor);
  output..write(kind)..write(' ');
  if (useColors) output.write(_NO_COLOR);
  if (span == null) {
    output.write(message);
  } else {
    output.write(span.getLocationMessage(message,
          useColors: useColors,
          color: levelColor));
  }
  return output.toString();
}

More details here:
https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/pkg/polymer/lib/src/build/linter.dart?r=31021#­190

[munificent]

Removed Type-Defect label.
Added Type-Enhancement label.

[munificent]

Removed Priority-Unassigned label.
Added Priority-Medium label.

[munificent]

Marked this as blocking #15829.

[munificent]

We've been doing this for a while, as far as I know.

---

Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#773.