add the spawn function for ES6 generators

examples/async-generators/3-spawn.js

@@ -0,0 +1,27 @@
+var Q = require('../../q');
+
+function delay(millis, answer) {
+    const deferredResult = Q.defer();
+    setTimeout(function() {
+        deferredResult.resolve(answer);
+    }, millis);
+    return deferredResult.promise;
+}
+
+function foo() {
+    return delay(1000, 5);
+}
+
+function bar() {
+    return delay(1000, 10);
+}
+
+Q.spawn(function*() {
+    var x = yield foo();
+    console.log(x);
+
+    var y = yield bar();
+    console.log(y);
+
+    console.log('result', x + y);
+});

q.js

@@ -1126,6 +1126,18 @@ function async(makeGenerator) {
     };
 }
 
+/**
+ * The spawn function is a small wrapper around async that immediately
+ * calls the generator and also ends the promise chain, so that any
+ * unhandled errors are thrown instead of forwarded to the error
+ * handler. This is useful because it's extremely common to run
+ * generators at the top-level to work with libraries.
+ */
+Q.spawn = spawn;
+function spawn(makeGenerator) {
+    Q.done(Q.async(makeGenerator)());
+}
+
 // FIXME: Remove this interface once ES6 generators are in SpiderMonkey.
 /**
  * Throws a ReturnValue exception to stop an asynchronous generator.