[vm/ffi] NNBD treatment of nullptr

[dcharkes]

We've given nullptr a Null type arg to make it easily assignable without casts to arbitrary type args.

sdk/sdk/lib/ffi/ffi.dart

Lines 31 to 33 in e7b1392

	/// Represents a pointer into the native C memory corresponding to "NULL", e.g.
	/// a pointer with address 0.
	final Pointer<Null> nullptr = Pointer.fromAddress(0);

sdk/tests/ffi/data_test.dart

Line 332 in e7b1392

Pointer<Int8> value = nullptr;

However, this is no longer allowed with NNBD.

ERROR|STATIC_TYPE_WARNING|INVALID_ASSIGNMENT|/usr/local/google/home/dacoharkes/dart-sdk/sdk/tests/ffi/data_test.dart|331|25|7|A value of type 'Pointer' can't be assigned to a variable of type 'Pointer'.

cc @mkustermann

Proposed solution is to change it to the following.

Pointer<T> nullptr<T extends NativeType>() => fromAddress<T>(0);

Pointer<Int8> value = nullptr();

However, this has some downsides:

• We cannot mark the current nullptr as deprecated, complicating migration.
• For what it does, a factory constructor would be better, but it would be more verbose.

(Note that we want to keep the API for the NNBD sdk and the legacy sdk compatible, so we also need to migrate the old sdk.)

[dcharkes]

cc @lrhn

[lrhn]

You probably want final nullptr = Pointer<Never>.fromAddress(0);.
I assume reading from address 0 will throw, which is exactly the correct behavior for a method returning Never.