Get rid of unnecessary flutter_root argument. (#9622)

packages/flutter_tools/bin/fuchsia_tester.dart

@@ -28,12 +28,10 @@ import '../lib/src/usage.dart';
 const String _kOptionPackages = "packages";
 const String _kOptionShell = "shell";
 const String _kOptionTestDirectory = "test-directory";
-const String _kOptionFlutterRoot = "flutter-root";
 const List<String> _kRequiredOptions = const <String>[
   _kOptionPackages,
   _kOptionShell,
   _kOptionTestDirectory,
-  _kOptionFlutterRoot,
 ];
 
 Future<Null> main(List<String> args) async {
@@ -65,16 +63,17 @@ Future<Null> run(List<String> args) async {
   final ArgParser parser = new ArgParser()
     ..addOption(_kOptionPackages, help: 'The .packages file')
     ..addOption(_kOptionShell, help: 'The Flutter shell binary')
-    ..addOption(_kOptionTestDirectory, help: 'Directory containing the tests')
-    ..addOption(_kOptionFlutterRoot, help: 'Flutter root');
+    ..addOption(_kOptionTestDirectory, help: 'Directory containing the tests');
   final ArgResults argResults = parser.parse(args);
   if (_kRequiredOptions
       .any((String option) => !argResults.options.contains(option))) {
     printError('Missing option! All options must be specified.');
     exit(1);
   }
-  // TODO(pylaligand): use a temp directory instead.
-  Cache.flutterRoot = argResults[_kOptionFlutterRoot];
+  final Directory tempDirectory =
+      fs.systemTempDirectory.createTempSync('fuchsia_tester');
+  try {
+    Cache.flutterRoot = tempDirectory.path;
     final Directory testDirectory =
         fs.directory(argResults[_kOptionTestDirectory]);
     final Iterable<String> tests = _findTests(testDirectory);
@@ -97,8 +96,8 @@ Future<Null> run(List<String> args) async {
     fs.currentDirectory = testDirectory;
 
     await test.main(testArgs);
-
-  if (exitCode != 0) {
     exit(exitCode);
+  } finally {
+    tempDirectory.deleteSync(recursive: true);
   }
 }