keep null check on virtual field

pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart

Index: pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart b/pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart
index 09e8649b3ca9a59af41ad540be323c497aa1979d..796811370f0062a4597812705ac9013670f8c09c 100644
--- a/pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart
+++ b/pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart
@@ -81,7 +81,8 @@ abstract class NullableTypeInference {
 
       if (element is PropertyAccessorElement && element.isGetter) {
         PropertyInducingElement variable = element.variable;
-        return variable.computeConstantValue()?.isNull ?? true;
+        var isVirtual = variable is FieldElement && variable.isVirtual;
+        return isVirtual || (variable.computeConstantValue()?.isNull ?? true);
       }
 
       // Other types of identifiers are nullable (parameters, fields).