Fix build_test README and improve a comment. (#4090)

Author: davidmorgan

build_test/README.md

@@ -84,11 +84,16 @@ write individual sources to it from a `PackageAssetReader`, or write all sources
 to it with `loadIsolateSources`:
 
 ```dart
-final readerWriter = TestReaderWriter();
-await readerWriter.loadIsolateSources();
+final readerWriter = TestReaderWriter(rootPackage: 'test_package');
+await readerWriter.testing.loadIsolateSources();
 testBuilder(
   yourBuilder,
-  {} /* test assets here */,
+  {'test_package|lib/a.dart': '''
+import 'package:real_package/annotations.dart';
+
+@RealAnnotation()
+class TestClass {}
+'''},
   readerWriter: readerWriter,
 );
 ```

build_test/lib/src/test_reader_writer.dart

@@ -16,6 +16,9 @@ import 'in_memory_reader_writer.dart';
 /// files.
 ///
 /// Writes and deletes are notified to [FakeWatcher].
+///
+/// You must pass a `rootPackage` if the `TestReaderWriter` will be used in
+/// a build. This specifies which package `build_runner` is running in.
 abstract interface class TestReaderWriter
     implements AssetReader, RunnerAssetWriter {
   factory TestReaderWriter({String? rootPackage}) =>