Roll Observatory packages and add a roll script

packages/petitparser/lib/src/debug/progress.dart

+part of petitparser.debug;
+
+/// Returns a transformed [parser] that when being used to read input
+/// visually prints its progress while progressing.
+///
+/// For example, the snippet
+///
+///     var parser = letter() & word().star();
+///     progress(parser).parse('f123');
+///
+/// produces the following output:
+///
+///     * Instance of 'SequenceParser'
+///     * Instance of 'CharacterParser'[letter expected]
+///     ** Instance of 'PossessiveRepeatingParser'[0..*]
+///     ** Instance of 'CharacterParser'[letter or digit expected]
+///     *** Instance of 'CharacterParser'[letter or digit expected]
+///     **** Instance of 'CharacterParser'[letter or digit expected]
+///     ***** Instance of 'CharacterParser'[letter or digit expected]
+///
+/// Jumps backwards mean that the parser is back-tracking. Often choices can
+/// be reordered to such expensive parses.
+Parser progress(Parser parser, [OutputHandler output = print]) {
+  return transformParser(parser, (each) {
+    return new ContinuationParser(each, (continuation, context) {
+      output('${_repeat(1 + context.position, '*')} $each');
+      return continuation(context);
+    });
+  });
+}