chore(a11y): Allow FocusTrap to be extended

This will be used to allow tabbing from edit lenses to another table cell in popover edit.

Author: kseamon

src/cdk/a11y/focus-trap/focus-trap.ts

@@ -37,8 +37,8 @@ export class FocusTrap {
   private _hasAttached = false;
 
   // Event listeners for the anchors. Need to be regular functions so that we can unbind them later.
-  private _startAnchorListener = () => this.focusLastTabbableElement();
-  private _endAnchorListener = () => this.focusFirstTabbableElement();
+  protected startAnchorListener = () => this.focusLastTabbableElement();
+  protected endAnchorListener = () => this.focusFirstTabbableElement();
 
   /** Whether the focus trap is active. */
   get enabled(): boolean { return this._enabled; }
@@ -70,15 +70,15 @@ export class FocusTrap {
     const endAnchor = this._endAnchor;
 
     if (startAnchor) {
-      startAnchor.removeEventListener('focus', this._startAnchorListener);
+      startAnchor.removeEventListener('focus', this.startAnchorListener);
 
       if (startAnchor.parentNode) {
         startAnchor.parentNode.removeChild(startAnchor);
       }
     }
 
     if (endAnchor) {
-      endAnchor.removeEventListener('focus', this._endAnchorListener);
+      endAnchor.removeEventListener('focus', this.endAnchorListener);
 
       if (endAnchor.parentNode) {
         endAnchor.parentNode.removeChild(endAnchor);
@@ -103,12 +103,12 @@ export class FocusTrap {
     this._ngZone.runOutsideAngular(() => {
       if (!this._startAnchor) {
         this._startAnchor = this._createAnchor();
-        this._startAnchor!.addEventListener('focus', this._startAnchorListener);
+        this._startAnchor!.addEventListener('focus', this.startAnchorListener);
       }
 
       if (!this._endAnchor) {
         this._endAnchor = this._createAnchor();
-        this._endAnchor!.addEventListener('focus', this._endAnchorListener);
+        this._endAnchor!.addEventListener('focus', this.endAnchorListener);
       }
     });
 
@@ -333,15 +333,10 @@ export class FocusTrap {
 /** Factory that allows easy instantiation of focus traps. */
 @Injectable({providedIn: 'root'})
 export class FocusTrapFactory {
-  private _document: Document;
-
   constructor(
       private _checker: InteractivityChecker,
       private _ngZone: NgZone,
-      @Inject(DOCUMENT) _document: any) {
-
-    this._document = _document;
-  }
+      @Inject(DOCUMENT) private _document: any) {}
 
   /**
    * Creates a focus-trapped region around the given element.