Add some Dart benchmarks running on fletch.

test/benchmark/fib.dart

@@ -0,0 +1,13 @@
+fib(n) {
+  if (n < 2) return n;
+  return fib(n - 1) + fib(n - 2);
+}
+
+main() {
+  Stopwatch watch = new Stopwatch();
+  watch.start();
+  for (var i = 0; i < 5; i++) {
+    print(fib(28));
+  }
+  print("elapsed: ${watch.elapsedMilliseconds / 1000}");
+}