Use handleEvent instead arrow function

src/focus.js

@@ -20,15 +20,6 @@ export class Focus {
     this.taskQueue = taskQueue;
     this.isAttached = false;
     this.needsApply = false;
-
-    this.focusListener = e => {
-      this.value = true;
-    };
-    this.blurListener = e => {
-      if (DOM.activeElement !== this.element) {
-        this.value = false;
-      }
-    };
   }
 
   /**
@@ -64,16 +55,34 @@ export class Focus {
       this.needsApply = false;
       this._apply();
     }
-    this.element.addEventListener('focus', this.focusListener);
-    this.element.addEventListener('blur', this.blurListener);
+    this.element.addEventListener('focus', this);
+    this.element.addEventListener('blur', this);
   }
 
   /**
   * Invoked when the attribute is detached from the DOM.
   */
   detached() {
     this.isAttached = false;
-    this.element.removeEventListener('focus', this.focusListener);
-    this.element.removeEventListener('blur', this.blurListener);
+    this.element.removeEventListener('focus', this);
+    this.element.removeEventListener('blur', this);
+  }
+
+  handleEvent(e) {
+    switch (e.type) {
+    case 'focus': this.handleFocus(e); break;
+    case 'blur': this.handleBlur(e); break;
+    default: break;
+    }
+  }
+
+  handleFocus(e) {
+    this.value = true;
+  }
+
+  handleBlur(e) {
+    if (DOM.activeElement !== this.element) {
+      this.value = false;
+    }
   }
 }