Resolve yield * issues for Firefox < 45 (#278)

* Detect when Firefox is trying to use the broken, alternative Iterator form and don't use it. Polyfill instead.

* Fix @@iterator function call for generators, so that the generator itself is returned.

packages/regenerator-runtime/runtime.js

@@ -416,6 +416,15 @@
 
   Gp[toStringTagSymbol] = "Generator";
 
+  // A Generator should always return itself as the iterator object when the
+  // @@iterator function is called on it. Some browsers' implementations of the
+  // iterator prototype chain incorrectly implement this, causing the Generator
+  // object to not be returned from this call. This ensures that doesn't happen.
+  // See https://github.com/facebook/regenerator/issues/274 for more details.
+  Gp[iteratorSymbol] = function() {
+    return this;
+  };
+
   Gp.toString = function() {
     return "[object Generator]";
   };