Simplify Filesystem, introduce TestReaderWriter (#3873)

* Introduce `TestReaderWriter` with test-specific API.

Move `FilesystemCache` and `AssetPathProvider` out of `Filesystem` and into
the `Reader` classes.

* Address review comments.

Author: davidmorgan

_test_common/lib/in_memory_reader_writer.dart

@@ -7,6 +7,8 @@ import 'dart:convert';
 import 'package:build/build.dart';
 import 'package:build_runner_core/build_runner_core.dart';
 import 'package:build_test/build_test.dart';
+// ignore: implementation_imports
+import 'package:build_test/src/in_memory_reader_writer.dart';
 import 'package:path/path.dart' as p;
 import 'package:watcher/watcher.dart';
 
@@ -26,7 +28,7 @@ class InMemoryRunnerAssetReaderWriter extends InMemoryAssetReaderWriter
 
   @override
   Future writeAsBytes(AssetId id, List<int> bytes) async {
-    var type = assets.containsKey(id) ? ChangeType.MODIFY : ChangeType.ADD;
+    var type = testing.exists(id) ? ChangeType.MODIFY : ChangeType.ADD;
     await super.writeAsBytes(id, bytes);
     FakeWatcher.notifyWatchers(
       WatchEvent(type, p.absolute(id.package, p.fromUri(id.path))),
@@ -39,7 +41,7 @@ class InMemoryRunnerAssetReaderWriter extends InMemoryAssetReaderWriter
     String contents, {
     Encoding encoding = utf8,
   }) async {
-    var type = assets.containsKey(id) ? ChangeType.MODIFY : ChangeType.ADD;
+    var type = testing.exists(id) ? ChangeType.MODIFY : ChangeType.ADD;
     await super.writeAsString(id, contents, encoding: encoding);
     FakeWatcher.notifyWatchers(
       WatchEvent(type, p.absolute(id.package, p.fromUri(id.path))),
@@ -49,7 +51,7 @@ class InMemoryRunnerAssetReaderWriter extends InMemoryAssetReaderWriter
   @override
   Future delete(AssetId id) async {
     onDelete?.call(id);
-    assets.remove(id);
+    testing.delete(id);
     FakeWatcher.notifyWatchers(
       WatchEvent(ChangeType.REMOVE, p.absolute(id.package, p.fromUri(id.path))),
     );

_test_common/lib/test_phases.dart

@@ -116,9 +116,9 @@ Future<TestBuildersResult> testBuilders(
   inputs.forEach((serializedId, contents) {
     var id = makeAssetId(serializedId);
     if (contents is String) {
-      readerWriter.filesystem.writeAsStringSync(id, contents);
+      readerWriter.testing.writeString(id, contents);
     } else if (contents is List<int>) {
-      readerWriter.filesystem.writeAsBytesSync(id, contents);
+      readerWriter.testing.writeBytes(id, contents);
     }
   });
 
@@ -179,7 +179,7 @@ Future<TestBuildersResult> testBuilders(
 void checkBuild(
   BuildResult result, {
   Map<String, Object>? outputs,
-  required InMemoryAssetReaderWriter readerWriter,
+  required TestReaderWriter readerWriter,
   BuildStatus status = BuildStatus.success,
   String rootPackage = 'a',
   String expectedGeneratedDir = 'generated',

build/lib/src/builder/build_step_impl.dart

@@ -100,6 +100,9 @@ class BuildStepImpl implements BuildStep, AssetReaderState {
   @override
   Filesystem get filesystem => _reader.filesystem;
 
+  @override
+  FilesystemCache get cache => _reader.cache;
+
   @override
   AssetFinder get assetFinder => _reader.assetFinder;