Add runtime utility module.

Author: benjamn

runtime.js

@@ -0,0 +1,24 @@
+exports.async = function(generator) {
+  return new Promise(function(resolve, reject) {
+    var callNext = step.bind(generator.next);
+    var callThrow = step.bind(generator.throw);
+
+    function step(arg) {
+      try {
+        var info = this(arg);
+        var value = info.value;
+      } catch (error) {
+        reject(error);
+        return;
+      }
+
+      if (info.done) {
+        resolve(value);
+      } else {
+        Promise.resolve(value).then(callNext, callThrow);
+      }
+    }
+
+    callNext();
+  });
+};