Support passing a file that contains a list of tests to run

If the final filename argument begins with an `@` it is a file (relative to cwd) that contains a list of test file/directories that should replace it in the arguments.

See Dart-Code/Dart-Code#5473

Author: DanTup

pkgs/test/CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 1.25.16-wip
 
+* Allow the final argument to be a filename prefixed with `@` that contains a
+  list of test files/directories to run. The lines of this file simply replace
+  the @file during argument parsing (ignoring blanks).
+
 ## 1.25.15
 
 * Allow the latest version of `package:shelf_web_socket`.

pkgs/test/test/runner/runner_test.dart

@@ -324,6 +324,15 @@ $_usage''');
       await test.shouldExit(1);
     });
 
+    test('a non-existent last @file is passed', () async {
+      var test = await runTest(['@mytestfile']);
+      expect(
+          test.stderr,
+          emitsThrough(contains(
+              'Failed to read the file "mytestfile" to read test paths')));
+      await test.shouldExit(64);
+    });
+
     // TODO(nweiz): test what happens when a test file is unreadable once issue
     // 15078 is fixed.
   });
@@ -407,6 +416,22 @@ $_usage''');
       expect(test.stdout, emitsThrough(contains('+1: All tests passed!')));
       await test.shouldExit(0);
     });
+
+    test('provided via a final @file argument', () async {
+      await d.file('test.dart', _success).create();
+      await d.file('mytestfiles.txt', 'test.dart').create();
+      var test = await runTest(['@mytestfiles.txt']);
+      expect(test.stdout, emitsThrough(contains('+1: All tests passed!')));
+      await test.shouldExit(0);
+    });
+
+    test('provided via a final @file argument ignoring empty lines', () async {
+      await d.file('test.dart', _success).create();
+      await d.file('mytestfiles.txt', '\n\r\ntest.dart\r\n').create();
+      var test = await runTest(['@mytestfiles.txt']);
+      expect(test.stdout, emitsThrough(contains('+1: All tests passed!')));
+      await test.shouldExit(0);
+    });
   });
 
   group('runs successful tests with async setup', () {

pkgs/test_core/lib/src/runner/configuration/args.dart

@@ -298,11 +298,32 @@ class _Parser {
       compilerSelections.add(CompilerSelection.parse('vm:source'));
     }
 
-    final paths = _options.rest.isEmpty ? null : _options.rest;
+    var paths = _options.rest.isEmpty ? null : _options.rest;
 
     Map<String, Set<TestSelection>>? selections;
     if (paths != null) {
       selections = {};
+
+      // The last path can start with @ and is a file that contains a list
+      // of paths that should be appended as if they were just inlined into
+      // the args. It is up to the calling code to clean up this file (if
+      // required) after the test process terminates.
+      var lastPath = paths.last;
+      if (lastPath.startsWith('@')) {
+        lastPath = lastPath.substring(1); // Strip @
+        paths = paths.toList(); // Make modifyable.
+        var argsFile = File(lastPath);
+        try {
+          paths.removeLast();
+          paths.addAll(argsFile
+              .readAsLinesSync()
+              .where((line) => line.trim().isNotEmpty));
+        } on FileSystemException catch (e) {
+          throw FormatException(
+              'Failed to read the file "$lastPath" to read test paths: $e');
+        }
+      }
+
       for (final path in paths) {
         _parseTestSelection(path, selections);
       }