Roll Observatory packages and add a roll script

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

+part of petitparser.debug;
+
+/// Returns a transformed [parser] that when being used to read input prints a
+/// trace of all activated parsers and their respective parse results.
+///
+/// For example, the snippet
+///
+///     var parser = letter() & word().star();
+///     trace(parser).parse('f1');
+///
+/// produces the following output:
+///
+///     Instance of 'SequenceParser'
+///       Instance of 'CharacterParser'[letter expected]
+///       Success[1:2]: f
+///       Instance of 'PossessiveRepeatingParser'[0..*]
+///         Instance of 'CharacterParser'[letter or digit expected]
+///         Success[1:3]: 1
+///         Instance of 'CharacterParser'[letter or digit expected]
+///         Failure[1:3]: letter or digit expected
+///       Success[1:3]: [1]
+///     Success[1:3]: [f, [1]]
+///
+/// Indentation signifies the activation of a parser object. Reverse indentation
+/// signifies the returning of a parse result either with a success or failure
+/// context.
+Parser trace(Parser parser, [OutputHandler output = print]) {
+  var level = 0;
+  return transformParser(parser, (each) {
+    return new ContinuationParser(each, (continuation, context) {
+      output('${_repeat(level, '  ')}$each');
+      level++;
+      var result = continuation(context);
+      level--;
+      output('${_repeat(level, '  ')}$result');
+      return result;
+    });
+  });
+}