Tests for `void`.

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index e2bc6c8bab58189f34bd06f817cd3c590ffb5ace..286abf8b205574c82ce0c2073b48baa54a40a98d 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1173,6 +1173,9 @@ class JavaScriptBackend extends Backend {
       } else if (cls == helpers.mapLiteralClass) {
         impactTransformer.registerBackendImpact(
             impactBuilder, impacts.mapLiteralClass);
+      } else if (cls == commonElements.objectClass) {
+        impactTransformer.registerBackendImpact(
+            impactBuilder, impacts.objectClass);
       }
     }
     if (cls == helpers.closureClass) {
@@ -3235,6 +3238,7 @@ class JavaScriptBackendClasses implements BackendClasses {
 
   JavaScriptBackendClasses(this.helpers);
 
+  ClassElement get voidClass => helpers.voidClass;
   ClassElement get intClass => helpers.jsIntClass;
   ClassElement get uint32Class => helpers.jsUInt32Class;
   ClassElement get uint31Class => helpers.jsUInt31Class;