Refactor FileBasedAssetReader and FileBasedAssetWriter to ReaderWriter.

[davidmorgan]

For #3811.

FileBasedAssetReader and FileBasedAssetWriter merge into ReaderWriter, and can now be used for testing too when an in-memory filesystem is provided, replacing most of InMemoryAssetReaderWriter. Finding assets is split out to classes InMemoryAssetFinder and PackageGraphAssetFinder to allow it to work for both cases.

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[jensjoha]

You assume below this is non-null. Shouldn't you mark it as such?

[jensjoha]

(and possibly overwrite the inputTracker getter to non-null to avoid some !s below)

[davidmorgan]

That's awkward because of the extends relationship, the super class copyWith wants it to be nullable. It could be covariant.

Really the awkward part is the test and non-test code doing different things, making it nullable for one and not for the other.

Fortunately the next PR addresses exactly this, unifying test and non-test use and moving InputTracker out of AssetReader entirely to the one place it's actually used :)

Thanks.

[jensjoha]

I don't think I understand... Why wouldn't this work:

diff --git a/build_test/lib/src/in_memory_reader_writer.dart b/build_test/lib/src/in_memory_reader_writer.dart
index f4ca7bdd..66e010f4 100644
--- a/build_test/lib/src/in_memory_reader_writer.dart
+++ b/build_test/lib/src/in_memory_reader_writer.dart
@@ -43,9 +43,12 @@ class InMemoryAssetReaderWriter extends ReaderWriter
     required super.assetPathProvider,
     required super.filesystem,
     required super.cache,
-    required super.inputTracker,
+    required InputTracker super.inputTracker,
   }) : super.using();
 
+  @override
+  InputTracker get inputTracker => super.inputTracker!;
+
   @override
   InMemoryAssetReaderWriter copyWith({
     AssetPathProvider? assetPathProvider,

I mean, with it going away it doesn't matter, but I don't understand the argument.

[davidmorgan]

Ah, you are correct.

The problem I mentioned only happens if inputTracker is a parameter to copyWith, but it no longer is. Sorry for the confusion, I was repeating compiler complaints from a stale state :)