Take a first pass at adding a benchmark.

benchmark/benchmark.dart

Index: benchmark/benchmark.dart
diff --git a/benchmark/benchmark.dart b/benchmark/benchmark.dart
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+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library markdown.benchmark.benchmark;
+
+import 'dart:io';
+
+import 'package:path/path.dart' as p;
+
+import 'package:markdown/markdown.dart';
+
+const numTrials = 100;
+const runsPerTrial = 50;
+
+final source = loadFile("input.md");
+final expected = loadFile("output.html");
+
+void main(List<String> args) {
+  var best = 99999999.0;

[nweiz, 2015/09/01 22:03:50]

double.INFINITY

[Bob Nystrom, 2015/09/01 23:59:40]

Done.

+
+  // Run the benchmark several times. This ensures the VM is warmed up and lets
+  // us see how much variance there is.
+  for (var i = 0; i <= numTrials; i++) {
+    var start = new DateTime.now();
+
+    // For a single benchmark, convert the source multiple times.
+    var result;
+    for (var j = 0; j < runsPerTrial; j++) {
+      result = markdownToHtml(source);
+    }
+
+    var elapsed =
+        new DateTime.now().difference(start).inMilliseconds / runsPerTrial;
+
+    // Keep track of the best run so far.
+    if (elapsed >= best) continue;
+    best = elapsed;
+
+    // Sanity check to make sure the output is what we expect and to make sure
+    // the VM doesn't optimize "dead" code away.
+    if (result != expected) {
+      print("Incorrect output:\n$result");
+      exit(1);
+    }
+
+    // Don't print the first run. It's always terrible since the VM hasn't
+    // warmed up yet.
+    if (i == 0) continue;
+    printResult("Run ${padLeft('#$i', 3)}", elapsed);
+  }
+
+  printResult("Best   ", best);
+}
+
+String loadFile(String name) {
+  var path = p.join(p.dirname(p.fromUri(Platform.script)), name);
+  return new File(path).readAsStringSync();
+}
+
+void printResult(String label, double time) {
+  print("$label: ${padLeft(time.toStringAsFixed(2), 4)}ms "
+      "${'=' * ((time * 20).toInt())}");
+}
+
+String padLeft(input, int length) {
+  var result = input.toString();
+  if (result.length < length) {
+    result = " " * (length - result.length) + result;
+  }
+
+  return result;
+}