Make requires lazy to improve startup times

index.js

@@ -3,11 +3,8 @@
 var path = require('path');
 var fs   = require('fs');
 var resolve = require('resolve');
-var MergeTrees = require('broccoli-merge-trees');
-var Funnel = require('broccoli-funnel');
-var BabelTranspiler = require('broccoli-babel-transpiler');
-var Concat = require('broccoli-concat');
-var VersionChecker = require('ember-cli-version-checker');
+
+var requireCache = {};
 
 module.exports = {
   name: 'Ember CLI QUnit',
@@ -33,6 +30,7 @@ module.exports = {
   init: function() {
     this._super.init && this._super.init.apply(this, arguments);
 
+    var VersionChecker = this.require('ember-cli-version-checker');
     var checker = new VersionChecker(this);
     var dep = checker.for('ember-cli', 'npm');
 
@@ -48,8 +46,27 @@ module.exports = {
     return path.join(__dirname, 'blueprints');
   },
 
+  require: function(name) {
+    if (!requireCache) {
+      requireCache = {};
+    }
+
+    var requiredModule = requireCache[name];
+
+    if (!requiredModule) {
+      requiredModule = requireCache[name] = require(name);
+    }
+
+    return requiredModule;
+  },
+
+  mergeTrees: function() {
+    var mergeTrees = this.require('broccoli-merge-trees');
+    return mergeTrees.apply(null, arguments);
+  },
+
   treeForAddonTestSupport: function() {
-    return new MergeTrees(this._getDependencyTrees());
+    return this.mergeTrees(this._getDependencyTrees());
   },
 
   treeForVendor: function(tree) {
@@ -66,14 +83,16 @@ module.exports = {
 
     if (this._shouldImportEmberQUnit) {
       // support for Ember CLI < 2.2.0-beta.1
-      var depTree = new MergeTrees(this._getDependencyTrees());
+      var depTree = this.mergeTrees(this._getDependencyTrees());
 
+      var BabelTranspiler = this.require('broccoli-babel-transpiler');
       var transpiled = new BabelTranspiler(depTree, {
         loose: true,
         moduleIds: true,
         modules: 'amdStrict'
       });
 
+      var Concat = this.require('broccoli-concat');
       var concattedTree = new Concat(transpiled, {
         inputFiles: ['**/*.js'],
         outputFile: '/ember-qunit/ember-qunit.js',
@@ -83,7 +102,7 @@ module.exports = {
       trees.push(concattedTree);
     }
 
-    return new MergeTrees(trees, {
+    return this.mergeTrees(trees, {
       annotation: 'ember-cli-qunit: treeForVendor'
     });
   },
@@ -154,6 +173,7 @@ module.exports = {
   },
 
   _notificationsTree: function() {
+    var Funnel = this.require('broccoli-funnel');
     var notificationsPath = path.dirname(resolve.sync('qunit-notifications'));
     return new Funnel(notificationsPath, {
       include: [ 'index.js' ],