Revert "Adding polyfill support for custom elements."

tools/dom/src/dartium_Platform.dart

Index: tools/dom/src/dartium_Platform.dart
diff --git a/tools/dom/src/dartium_Platform.dart b/tools/dom/src/dartium_Platform.dart
index 48f323837dd3e2039e5f39265a8088a062454bef..a851ccab25689859275adf32f88548bedd459f39 100644
--- a/tools/dom/src/dartium_Platform.dart
+++ b/tools/dom/src/dartium_Platform.dart
@@ -18,14 +18,4 @@ class Platform {
    * error.
    */
   static final supportsSimd = true;
-
-  /**
-   * 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.
-   */
-  void upgradeCustomElements(Node node) {
-    // no-op, provided for dart2js polyfill.
-  }
 }