Backport - Bug 1440321 - Convert Task.jsm to async/await in devtools/client. r=jryans

Author: jasonLaster

src/test/mochitest/head.js

@@ -39,6 +39,8 @@ Services.scriptloader.loadSubScript(
 );
 
 var { Toolbox } = require("devtools/client/framework/toolbox");
+var { Task } = require("devtools/shared/task");
+
 const sourceUtils = {
   isLoaded: source => source.get("loadedState") === "loaded"
 };