[tool] Move changed file detection to base command class (flutter#8730)

Consolidates the code to find all changed file paths into the `PackageLoopingCommand` class that is the base of almost all of the repo tooling commands. This in a preparatory PR for a future change to allow each command to define a list of files or file patterns that definitively *don't* affect that test, so that CI can be smarter about what tests to run (e.g., not running expensive integration tests for README changes).

A side effect of this change is that tests of almost all commands now need a mock `GitDir` instance. This would add a lot of copy/pasted boilerplate to the test setup, and there is already too much of that, so instead this refactors common test setup:
- Creating a memory file system
- Populating it with a packages directory
- Creating a RecordingProcessRunner to mock out process calls
- Creating a mock GitDir that forwards to a RecordingProcessRunner

into a helper method (using records and destructuring to easily return multiple values). While some tests don't need all of these steps, those that don't can easily ignore parts of it, and it will make it much easier to update tests in the future if they need them, and it makes the setup much more consistent which makes it easier to reason about test setup in general.

Prep for flutter/flutter#136394

Author: stuartmorgan-g

script/tool/lib/src/analyze_command.dart

@@ -17,6 +17,7 @@ class AnalyzeCommand extends PackageLoopingCommand {
     super.packagesDir, {
     super.processRunner,
     super.platform,
+    super.gitDir,
   }) {
     argParser.addMultiOption(_customAnalysisFlag,
         help:

script/tool/lib/src/build_examples_command.dart

@@ -48,6 +48,7 @@ class BuildExamplesCommand extends PackageLoopingCommand {
     super.packagesDir, {
     super.processRunner,
     super.platform,
+    super.gitDir,
   }) {
     argParser.addFlag(platformLinux);
     argParser.addFlag(platformMacOS);

script/tool/lib/src/common/git_version_finder.dart

@@ -28,15 +28,6 @@ class GitVersionFinder {
   /// The base branche used to find a merge point if baseSha is not provided.
   final String _baseBranch;
 
-  static bool _isPubspec(String file) {
-    return file.trim().endsWith('pubspec.yaml');
-  }
-
-  /// Get a list of all the pubspec.yaml file that is changed.
-  Future<List<String>> getChangedPubSpecs() async {
-    return (await getChangedFiles()).where(_isPubspec).toList();
-  }
-
   /// Get a list of all the changed files.
   Future<List<String>> getChangedFiles(
       {bool includeUncommitted = false}) async {

script/tool/lib/src/common/package_looping_command.dart

@@ -9,6 +9,7 @@ import 'package:path/path.dart' as p;
 import 'package:pub_semver/pub_semver.dart';
 
 import 'core.dart';
+import 'git_version_finder.dart';
 import 'output_utils.dart';
 import 'package_command.dart';
 import 'repository_package.dart';
@@ -226,6 +227,17 @@ abstract class PackageLoopingCommand extends PackageCommand {
   /// The suggested indentation for printed output.
   String get indentation => hasLongOutput ? '' : '  ';
 
+  /// The base SHA used to calculate changed files.
+  ///
+  /// This is guaranteed to be populated before [initializeRun] is called.
+  late String baseSha;
+
+  /// The repo-relative paths (using Posix separators) of all files changed
+  /// relative to [baseSha].
+  ///
+  /// This is guaranteed to be populated before [initializeRun] is called.
+  late List<String> changedFiles;
+
   // ----------------------------------------
 
   @override
@@ -264,6 +276,11 @@ abstract class PackageLoopingCommand extends PackageCommand {
 
     final DateTime runStart = DateTime.now();
 
+    // Populate the list of changed files for subclasses to use.
+    final GitVersionFinder gitVersionFinder = await retrieveVersionFinder();
+    baseSha = await gitVersionFinder.getBaseSha();
+    changedFiles = await gitVersionFinder.getChangedFiles();
+
     await initializeRun();
 
     final List<PackageEnumerationEntry> targetPackages =

script/tool/lib/src/custom_test_command.dart

@@ -21,6 +21,7 @@ class CustomTestCommand extends PackageLoopingCommand {
     super.packagesDir, {
     super.processRunner,
     super.platform,
+    super.gitDir,
   });
 
   @override

script/tool/lib/src/dart_test_command.dart

@@ -27,6 +27,7 @@ class DartTestCommand extends PackageLoopingCommand {
     super.packagesDir, {
     super.processRunner,
     super.platform,
+    super.gitDir,
   }) {
     argParser.addOption(
       kEnableExperiment,

script/tool/lib/src/drive_examples_command.dart

@@ -27,6 +27,7 @@ class DriveExamplesCommand extends PackageLoopingCommand {
     super.packagesDir, {
     super.processRunner,
     super.platform,
+    super.gitDir,
   }) {
     argParser.addFlag(platformAndroid,
         help: 'Runs the Android implementation of the examples',

script/tool/lib/src/federation_safety_check_command.dart

@@ -59,9 +59,8 @@ class FederationSafetyCheckCommand extends PackageLoopingCommand {
   @override
   Future<void> initializeRun() async {
     final GitVersionFinder gitVersionFinder = await retrieveVersionFinder();
-    final String baseSha = await gitVersionFinder.getBaseSha();
     print('Validating changes relative to "$baseSha"\n');
-    for (final String path in await gitVersionFinder.getChangedFiles()) {
+    for (final String path in changedFiles) {
       // Git output always uses Posix paths.
       final List<String> allComponents = p.posix.split(path);
       final int packageIndex = allComponents.indexOf('packages');

script/tool/lib/src/fetch_deps_command.dart

@@ -27,6 +27,7 @@ class FetchDepsCommand extends PackageLoopingCommand {
     super.packagesDir, {
     super.processRunner,
     super.platform,
+    super.gitDir,
   }) {
     argParser.addFlag(_dartFlag, defaultsTo: true, help: 'Run "pub get"');
     argParser.addFlag(_supportingTargetPlatformsOnlyFlag,

script/tool/lib/src/firebase_test_lab_command.dart

@@ -23,6 +23,7 @@ class FirebaseTestLabCommand extends PackageLoopingCommand {
     super.packagesDir, {
     super.processRunner,
     super.platform,
+    super.gitDir,
   }) {
     argParser.addOption(
       _gCloudProjectArg,