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

pkg/compiler/lib/src/inferrer/type_graph_nodes.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
index 2fe9245319374dbfa539ec6247628f2544935d31..403716fc20b19a040bc038a3868bf1f4efa2b3c6 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
@@ -1753,13 +1753,14 @@ TypeMask _narrowType(
     {bool isNullable: true}) {
   if (annotation.treatAsDynamic) return type;
   if (annotation.isObject) return type;
-  if (annotation.isVoid) return type;
   TypeMask otherType;
   if (annotation.isTypedef || annotation.isFunctionType) {
     otherType = closedWorld.commonMasks.functionType;
   } else if (annotation.isTypeVariable) {
     // TODO(ngeoffray): Narrow to bound.
     return type;
+  } else if (annotation.isVoid) {
+    otherType = closedWorld.commonMasks.nullType;
   } else {
     ResolutionInterfaceType interfaceType = annotation;
     otherType = new TypeMask.nonNullSubtype(interfaceType.element, closedWorld);