Remove Platform.upgrateCustomElements.

tools/dom/src/dart2js_Platform.dart

Index: tools/dom/src/dart2js_Platform.dart
diff --git a/tools/dom/src/dart2js_Platform.dart b/tools/dom/src/dart2js_Platform.dart
index ea50a6ae018d4c8d7f849c496f087c9516fd7653..5988011c0cd3c1c048605dcb22c0a3c7360ff25e 100644
--- a/tools/dom/src/dart2js_Platform.dart
+++ b/tools/dom/src/dart2js_Platform.dart
@@ -18,17 +18,4 @@ class Platform {
    * error.
    */
   static final supportsSimd = false;
-
-  /**
-   * Upgrade all custom elements in the subtree which have not been upgraded.
-   *
-   * This is needed to cover timing scenarios which the custom element polyfill
-   * does not cover.
-   */
-  static void upgradeCustomElements(Node node) {
-    if (JS('bool', '(#.CustomElements && #.CustomElements.upgradeAll)',
-        window, window)) {
-      JS('', '#.CustomElements.upgradeAll(#)', window, node);
-    }
-  }
 }