Use computeConstantValue instead of constantValue

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 39345037a4f893b90b4e543e92423f8338cb89e2..09e8649b3ca9a59af41ad540be323c497aa1979d 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 +81,7 @@ abstract class NullableTypeInference {
 
       if (element is PropertyAccessorElement && element.isGetter) {
         PropertyInducingElement variable = element.variable;
-        return variable.constantValue?.isNull ?? true;
+        return variable.computeConstantValue()?.isNull ?? true;
       }
 
       // Other types of identifiers are nullable (parameters, fields).