diff --git a/packages/core/src/util/ng_dev_mode.ts b/packages/core/src/util/ng_dev_mode.ts
index d078be0483..eeee40f078 100644
--- a/packages/core/src/util/ng_dev_mode.ts
+++ b/packages/core/src/util/ng_dev_mode.ts
@@ -8,6 +8,9 @@
 
 import {global} from './global';
 
+// Do not remove: needed for closure to be able to properly tree-shake ngDevMode.
+// goog.define
+
 declare global {
   /**
    * Values of ngDevMode
diff --git a/packages/core/src/util/ng_i18n_closure_mode.ts b/packages/core/src/util/ng_i18n_closure_mode.ts
index a966565819..5e784defdf 100644
--- a/packages/core/src/util/ng_i18n_closure_mode.ts
+++ b/packages/core/src/util/ng_i18n_closure_mode.ts
@@ -8,6 +8,9 @@
 
 import {global} from './global';
 
+// Do not remove: needed for closure to be able to properly tree-shake ngI18nClosureMode.
+// goog.define
+
 declare global {
   const ngI18nClosureMode: boolean;
 }