diff --git a/build-system/global-configs/experiments-config.json b/build-system/global-configs/experiments-config.json
index 87bf4d4f9bda..f061431de029 100644
--- a/build-system/global-configs/experiments-config.json
+++ b/build-system/global-configs/experiments-config.json
@@ -1,12 +1,5 @@
 {
   "experimentA": {},
   "experimentB": {},
-  "experimentC": {
-    "name": "Native Custom Elements V1",
-    "environment": "AMP",
-    "command": "gulp dist --defineExperimentConstant=NATIVE_CUSTOM_ELEMENTS_V1",
-    "issue": "https://github.com/ampproject/amphtml/issues/17243",
-    "expirationDateUTC": "2020-01-01",
-    "defineExperimentConstant": "NATIVE_CUSTOM_ELEMENTS_V1"
-  }
+  "experimentC": {}
 }
diff --git a/src/friendly-iframe-embed.js b/src/friendly-iframe-embed.js
index 95a5d4360448..0e6a3071d8b0 100644
--- a/src/friendly-iframe-embed.js
+++ b/src/friendly-iframe-embed.js
@@ -866,10 +866,7 @@ function installPolyfillsInChildWindow(parentWin, childWin) {
   installDOMTokenList(childWin);
   // The anonymous class parameter allows us to detect native classes vs
   // transpiled classes.
-  installCustomElements(
-    childWin,
-    NATIVE_CUSTOM_ELEMENTS_V1 ? class {} : undefined
-  );
+  installCustomElements(childWin, class {});
 }
 
 /**
diff --git a/src/polyfills.js b/src/polyfills.js
index 33dcda59261e..9aa164b8e1b0 100644
--- a/src/polyfills.js
+++ b/src/polyfills.js
@@ -41,7 +41,7 @@ if (self.document) {
   installGetBoundingClientRect(self);
   // The anonymous class parameter allows us to detect native classes vs
   // transpiled classes.
-  installCustomElements(self, NATIVE_CUSTOM_ELEMENTS_V1 ? class {} : undefined);
+  installCustomElements(self, class {});
 }
 
 // TODO(#18268, erwinm): For whatever reason imports to modules that have no
diff --git a/src/polyfills/custom-elements.js b/src/polyfills/custom-elements.js
index 46b9c82ba391..599b9ac3d314 100644
--- a/src/polyfills/custom-elements.js
+++ b/src/polyfills/custom-elements.js
@@ -937,9 +937,9 @@ export function copyProperties(obj, prototype) {
  * done.
  *
  * @param {!Window} win
- * @param {!Function=} opt_ctor
+ * @param {!Function} ctor
  */
-export function install(win, opt_ctor) {
+export function install(win, ctor) {
   // Don't install in no-DOM environments e.g. worker.
   const shouldInstall = win.document;
   const hasCE = hasCustomElements(win);
@@ -950,7 +950,7 @@ export function install(win, opt_ctor) {
   let install = true;
   let installWrapper = false;
 
-  if (opt_ctor && hasCE) {
+  if (ctor && hasCE) {
     // If ctor is constructable without new, it's a function. That means it was
     // compiled down, and we need to do the minimal polyfill because all you
     // cannot extend HTMLElement without native classes.
@@ -958,15 +958,16 @@ export function install(win, opt_ctor) {
       const {Reflect} = win;
 
       // "Construct" ctor using ES5 idioms
-      const instance = /** @type {!Function} */ (Object.create(
-        opt_ctor.prototype
-      ));
+      // I'm not sure why, but Closure will complain at the
+      // `Function.call.call()` below unless we cast to a Function instance
+      // here.
+      const instance = /** @type {!Function} */ (Object.create(ctor.prototype));
 
       // This will throw an error unless we're in a transpiled environemnt.
       // Native classes must be called as `new Ctor`, not `Ctor.call(instance)`.
       // We use `Function.call.call` because Closure is too smart for regular
       // `Ctor.call`.
-      Function.call.call(opt_ctor, instance);
+      Function.call.call(ctor, instance);
 
       // If that didn't throw, we're transpiled.
       // Let's find out if we can wrap HTMLElement and avoid a full patch.
diff --git a/testing/describes.js b/testing/describes.js
index 45cfb27ff3b8..67fd66f206e7 100644
--- a/testing/describes.js
+++ b/testing/describes.js
@@ -648,10 +648,7 @@ class RealWinFixture {
         } else {
           // The anonymous class parameter allows us to detect native classes
           // vs transpiled classes.
-          installCustomElements(
-            win,
-            NATIVE_CUSTOM_ELEMENTS_V1 ? class {} : undefined
-          );
+          installCustomElements(win, class {});
         }
 
         // Intercept event listeners
diff --git a/testing/iframe.js b/testing/iframe.js
index 6e4f5c31b849..5aa6a38173f3 100644
--- a/testing/iframe.js
+++ b/testing/iframe.js
@@ -245,10 +245,7 @@ export function createIframePromise(opt_runtimeOff, opt_beforeLayoutCallback) {
       installRuntimeServices(iframe.contentWindow);
       // The anonymous class parameter allows us to detect native classes vs
       // transpiled classes.
-      installCustomElements(
-        iframe.contentWindow,
-        NATIVE_CUSTOM_ELEMENTS_V1 ? class {} : undefined
-      );
+      installCustomElements(iframe.contentWindow, class {});
       installAmpdocServices(ampdoc);
       Services.resourcesForDoc(ampdoc).ampInitComplete();
       // Act like no other elements were loaded by default.