Revert "Void is not required to be `null` anymore."

pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart

Index: pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
index d239c1aa9b7a0e027fe0d8ad08b697e3a762555a..571e10616765ff66da36876abf45b119b94b7f91 100644
--- a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
@@ -117,6 +117,7 @@ class CheckedModeHelpers {
   /// All the checked mode helpers.
   static const List<CheckedModeHelper> helpers = const <CheckedModeHelper>[
     const MalformedCheckedModeHelper('checkMalformedType'),
+    const CheckedModeHelper('voidTypeCheck'),
     const CheckedModeHelper('stringTypeCast'),
     const CheckedModeHelper('stringTypeCheck'),
     const CheckedModeHelper('doubleTypeCast'),
@@ -202,6 +203,12 @@ class CheckedModeHelpers {
       return 'checkMalformedType';
     }
 
+    if (type.isVoid) {
+      assert(!typeCast); // Cannot cast to void.
+      if (nativeCheckOnly) return null;
+      return 'voidTypeCheck';
+    }
+
     if (type.isTypeVariable) {
       return typeCast
           ? 'subtypeOfRuntimeTypeCast'