memoize existsSync calls on the fastboot instance itself

In large applications with many nested addons and engines, this can do
excessive work.

Luckily it is perfectly consistent for us to cache this on the addon
instance state. In-fact it is incoherent for this to change from the
perspective of one fastboot addon instance

index.js

@@ -39,6 +39,18 @@ module.exports = {
 
   init() {
     this._super.init && this._super.init.apply(this, arguments);
+    this._existsCache = new Map();
+  },
+
+  existsSync(path) {
+    if (this._existsCache.has(path)) {
+      return this._existsCache.get(path);
+    }
+
+    const exists = existsSync(path);
+
+    this._existsCache.set(path, exists);
+    return exists;
   },
 
   /**
@@ -139,7 +151,7 @@ module.exports = {
     const currentAddonFastbootPath = path.join(addon.root, 'fastboot');
 
     let fastbootTree;
-    if (existsSync(currentAddonFastbootPath)) {
+    if (this.existsSync(currentAddonFastbootPath)) {
       fastbootTree = this.treeGenerator(currentAddonFastbootPath);
     }
 
@@ -168,7 +180,7 @@ module.exports = {
 
     // check the parent containing the fastboot directory
     const projectFastbootPath = path.join(this.project.root, 'fastboot');
-    if (existsSync(projectFastbootPath)) {
+    if (this.existsSync(projectFastbootPath)) {
       let fastbootTree = this.treeGenerator(projectFastbootPath);
       fastbootTrees.push(fastbootTree);
     }