Ignore EOF newline characters and added tests to dir_contents_diff tool (flutter/engine#50805)

No issue filed, but was [discussed on discord](https://discord.com/channels/608014603317936148/608021010377080866/1209568840644567091).

engine/src/flutter/testing/run_tests.py

@@ -922,6 +922,7 @@ def build_dart_host_test_list(build_dir):
               os.path.join(build_dir, 'dart-sdk', 'lib', 'libraries.json'),
           ],
       ),
+      (os.path.join('flutter', 'tools', 'dir_contents_diff'), []),
       (os.path.join('flutter', 'tools', 'engine_tool'), []),
       (os.path.join('flutter', 'tools', 'githooks'), []),
       (os.path.join('flutter', 'tools', 'header_guard_check'), []),

engine/src/flutter/tools/dir_contents_diff/lib/dir_contents_diff.dart

@@ -70,8 +70,20 @@ int dirContentsDiff(String goldenPath, String dirPath) {
     final String dirListing = _generateDirListing(dirPath);
     tempFile.writeAsStringSync(dirListing);
     final ProcessResult diffResult = Process.runSync(
-        'git', <String>['diff', '-p', goldenPath, tempFile.path],
-        runInShell: true, stdoutEncoding: utf8);
+      'git',
+      <String>[
+        'diff',
+        // If you manually edit the golden file, many text editors will add
+        // trailing whitespace. This flag ignores that because honestly it's
+        // not a significant part of this test.
+        '--ignore-space-at-eol',
+        '-p',
+        goldenPath,
+        tempFile.path,
+      ],
+      runInShell: true,
+      stdoutEncoding: utf8,
+    );
     if (diffResult.exitCode != 0) {
       print(
           'Unexpected diff in $goldenPath, use `git apply` with the following patch.\n');

engine/src/flutter/tools/dir_contents_diff/pubspec.yaml

@@ -1,3 +1,19 @@
 name: dir_contents_diff
 environment:
   sdk: '>=3.2.0-0 <4.0.0'
+dev_dependencies:
+  litetest: any
+  path: any
+dependency_overrides:
+  async_helper:
+    path: ../../../third_party/dart/pkg/async_helper
+  expect:
+    path: ../../../third_party/dart/pkg/expect
+  litetest:
+    path: ../../testing/litetest
+  meta:
+    path: ../../../third_party/dart/pkg/meta
+  path:
+    path: ../../../third_party/dart/third_party/pkg/path
+  smith:
+    path: ../../../third_party/dart/pkg/smith

engine/src/flutter/tools/dir_contents_diff/test/dir_contents_diff_test.dart

@@ -0,0 +1,77 @@
+import 'dart:io' as io;
+
+import 'package:litetest/litetest.dart';
+import 'package:path/path.dart' as p;
+
+void main() {
+  // Find a path to `dir_contents_diff.dart` from the working directory.
+  final String pkgPath = io.File.fromUri(io.Platform.script).parent.parent.path;
+  final String binPath = p.join(
+    pkgPath,
+    'bin',
+    'dir_contents_diff.dart',
+  );
+
+  // As a sanity check, ensure that the file exists.
+  if (!io.File(binPath).existsSync()) {
+    io.stderr.writeln('Unable to find $binPath');
+    io.exitCode = 1;
+    return;
+  }
+
+
+  // Runs `../bin/dir_contents_diff.dart` with the given arguments.
+  (int, String) runSync(String goldenPath, String dirPath) {
+    final io.ProcessResult result = io.Process.runSync(
+      'dart',
+      <String>[binPath, goldenPath, dirPath],
+    );
+    return (result.exitCode, result.stdout ?? result.stderr);
+  }
+
+  test('lists files and diffs successfully', () {
+    final String goldenPath = p.join(pkgPath, 'test', 'file_ok.txt');
+    final String dirPath = p.join(pkgPath, 'test', 'fixtures');
+    final (int exitCode, String output) = runSync(goldenPath, dirPath);
+    if (exitCode != 0) {
+      io.stderr.writeln('Expected exit code 0, got $exitCode');
+      io.stderr.writeln(output);
+    }
+    expect(exitCode, 0);
+  });
+
+  test('lists files and diffs successfully, even with an EOF newline', () {
+    final String goldenPath = p.join(pkgPath, 'test', 'file_ok_eof_newline.txt');
+    final String dirPath = p.join(pkgPath, 'test', 'fixtures');
+    final (int exitCode, String output) = runSync(goldenPath, dirPath);
+    if (exitCode != 0) {
+      io.stderr.writeln('Expected exit code 0, got $exitCode');
+      io.stderr.writeln(output);
+    }
+    expect(exitCode, 0);
+  });
+
+  test('diff fails when an expected file is missing', () {
+    final String goldenPath = p.join(pkgPath, 'test', 'file_bad_missing.txt');
+    final String dirPath = p.join(pkgPath, 'test', 'fixtures');
+    final (int exitCode, String output) = runSync(goldenPath, dirPath);
+    if (exitCode == 0) {
+      io.stderr.writeln('Expected non-zero exit code, got $exitCode');
+      io.stderr.writeln(output);
+    }
+    expect(exitCode, 1);
+    expect(output, contains('+a.txt'));
+  });
+
+  test('diff fails when an unexpected file is present', () {
+    final String goldenPath = p.join(pkgPath, 'test', 'file_bad_unexpected.txt');
+    final String dirPath = p.join(pkgPath, 'test', 'fixtures');
+    final (int exitCode, String output) = runSync(goldenPath, dirPath);
+    if (exitCode == 0) {
+      io.stderr.writeln('Expected non-zero exit code, got $exitCode');
+      io.stderr.writeln(output);
+    }
+    expect(exitCode, 1);
+    expect(output, contains('-c.txt'));
+  });
+}

engine/src/flutter/tools/dir_contents_diff/test/file_bad_missing.txt

@@ -0,0 +1 @@
+b.txt

engine/src/flutter/tools/dir_contents_diff/test/file_bad_unexpected.txt

@@ -0,0 +1,3 @@
+a.txt
+b.txt
+c.txt

engine/src/flutter/tools/dir_contents_diff/test/file_ok.txt

@@ -0,0 +1,2 @@
+a.txt
+b.txt

engine/src/flutter/tools/dir_contents_diff/test/file_ok_eof_newline.txt

@@ -0,0 +1,2 @@
+a.txt
+b.txt

engine/src/flutter/tools/dir_contents_diff/test/fixtures/a.txt

@@ -0,0 +1 @@
+Will be referenced by a test.

engine/src/flutter/tools/dir_contents_diff/test/fixtures/b.txt

@@ -0,0 +1 @@
+Will be referenced by a test.