refactor(cdk/a11y): clean up IE workarounds (#23304)

Cleans up the workarounds for IE11 from the `cdk/a11y` package.

Author: crisbeto

src/cdk/a11y/focus-trap/event-listener-inert-strategy.ts

@@ -8,7 +8,6 @@
 
 import {FocusTrapInertStrategy} from './focus-trap-inert-strategy';
 import {ConfigurableFocusTrap} from './configurable-focus-trap';
-import {closest} from './polyfill';
 
 /**
  * Lightweight FocusTrapInertStrategy that adds a document focus event
@@ -53,7 +52,8 @@ export class EventListenerFocusTrapInertStrategy implements FocusTrapInertStrate
 
     // Don't refocus if target was in an overlay, because the overlay might be associated
     // with an element inside the FocusTrap, ex. mat-select.
-    if (!focusTrapRoot.contains(target) && closest(target, 'div.cdk-overlay-pane') === null) {
+    if (target && !focusTrapRoot.contains(target) &&
+        target.closest('div.cdk-overlay-pane') === null) {
         // Some legacy FocusTrap usages have logic that focuses some element on the page
         // just before FocusTrap is destroyed. For backwards compatibility, wait
         // to be sure FocusTrap is still enabled before refocusing.

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

@@ -271,12 +271,10 @@ export class FocusTrap {
       return root;
     }
 
-    // Iterate in DOM order. Note that IE doesn't have `children` for SVG so we fall
-    // back to `childNodes` which includes text nodes, comments etc.
-    let children = root.children || root.childNodes;
+    const children = root.children;
 
     for (let i = 0; i < children.length; i++) {
-      let tabbableChild = children[i].nodeType === this._document.ELEMENT_NODE ?
+      const tabbableChild = children[i].nodeType === this._document.ELEMENT_NODE ?
         this._getFirstTabbableElement(children[i] as HTMLElement) :
         null;
 
@@ -295,10 +293,10 @@ export class FocusTrap {
     }
 
     // Iterate in reverse DOM order.
-    let children = root.children || root.childNodes;
+    const children = root.children;
 
     for (let i = children.length - 1; i >= 0; i--) {
-      let tabbableChild = children[i].nodeType === this._document.ELEMENT_NODE ?
+      const tabbableChild = children[i].nodeType === this._document.ELEMENT_NODE ?
         this._getLastTabbableElement(children[i] as HTMLElement) :
         null;