fixed bug, 'dartDeveloperLibrary()._extensions.keys' is not a function

Change-Id: I95a59575e3dfebf4e44008d171226994bb63ccba
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/399784
Commit-Queue: Srujan Gaddam <[email protected]>
Reviewed-by: Srujan Gaddam <[email protected]>
Commit-Queue: Jessy Yameogo <[email protected]>

Author: jyameo

pkg/dev_compiler/lib/js/ddc/ddc_module_loader.js

@@ -1913,7 +1913,7 @@ if (!self.deferred_loader) {
      * @returns {Array<string>} Array containing the extension names.
      */
     get extensionNames() {
-      return dartDeveloperLibrary()._extensions.keys().toList();
+      return dartDeveloperLibrary()._extensions.keys.toList();
     }
 
     /**