Revert "dart2js: fix for https://github.com/dart-lang/sdk/issues/24412"

pkg/compiler/lib/src/js_backend/codegen/glue.dart

Index: pkg/compiler/lib/src/js_backend/codegen/glue.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/glue.dart b/pkg/compiler/lib/src/js_backend/codegen/glue.dart
index a1d423323e7a1667a6333fd06ef645d15587cd47..37dd0ae333038226b49027cb39d9ab9ec0e54bbc 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/glue.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/glue.dart
@@ -261,11 +261,6 @@ class Glue {
   ClassElement get jsExtendableArrayClass => _backend.jsExtendableArrayClass;
   ClassElement get jsMutableArrayClass => _backend.jsMutableArrayClass;
 
-  ClassElement get jsPlainJavaScriptObjectClass =>
-      _backend.jsPlainJavaScriptObjectClass;
-  ClassElement get jsUnknownJavaScriptObjectClass =>
-      _backend.jsUnknownJavaScriptObjectClass;
-
   bool isStringClass(ClassElement classElement) =>
       classElement == _backend.jsStringClass ||
       classElement == _compiler.stringClass;