In the Dart web platform libraries, e.g. dart:html, there are APIs that depend on JS interoperability. With null-safety, the types returned from these interop procedures are not null-checked by default. In both DDC and dart2js, there exists a flag to turn on checks, or native null assertions, for these instances. In DDC, it‘s called nativeNonNullAsserts and in dart2js, it’s called --native-null-assertions.
Specifically, the flag focused on two cases. The first case is checks around types returned from APIs that are declared native. If the return type here is non-nullable, enabling native null assertions will throw an error if a null value is returned. For example,
int get foo native;
will throw an error if .foo returns a null value. This may happen due to a number of reasons, one of which could be browser incompatibility.
The second case is on JS() invocations. JS() is an internal-only function that allows you to inline JavaScript. If the static type of the JS() invocation is non-nullable, but a null value is returned, there will be an error thrown if native null assertions are enabled.
The goals with these native null assertions are to ensure the Dart web platform libraries are typed correctly and to help achieve sound null-safety.
If you come across an error related to this flag, this may or may not be a bug in the Dart web platform libraries. If so, please file a bug at: https://github.com/dart-lang/sdk/issues/labels/web-libraries
Native null assertions will be turned on by default across different build systems. If it is enabled, here's how you can disable it in the following build systems:
https://github.com/dart-lang/build/tree/master/docs/native_null_assertions.md