refactor: transparently resolve traversal invocation arg promises

packages/behavior/context/index.js

@@ -85,7 +85,23 @@ class Context extends Map {
         return node[step.key];
       case 'invoke':
         const args = step.args.map((arg) => arg.get(this));
-        return fastApply(previousNode, node, args);
+        // Any arg promises will be resolved; the arg values will be passed
+        // transparently to the invocation.
+        let hasPromise = false;
+        for (let i = 0; i < args.length; i++) {
+          if (args[i] instanceof Promise) {
+            hasPromise = true;
+            break;
+          }
+        }
+        if (hasPromise) {
+          return Promise.all(args).then((args) => {
+            return fastApply(previousNode, node, args);
+          });
+        }
+        else {
+          return fastApply(previousNode, node, args);
+        }
     }
   }