Merge pull request #765 from mzgoddard/execute-is-promise-deopt

Write src/engine/execute so it can be optimized

src/engine/execute.js

@@ -15,7 +15,11 @@ const blockUtility = new BlockUtility();
  * @return {boolean} True if the value appears to be a Promise.
  */
 const isPromise = function (value) {
-    return value && value.then && typeof value.then === 'function';
+    return (
+        value !== null &&
+        typeof value === 'object' &&
+        typeof value.then === 'function'
+    );
 };
 
 /**