Implement `flutter test -j` (#20493)

packages/flutter_tools/bin/fuchsia_tester.dart

diff --git a/packages/flutter_tools/bin/fuchsia_tester.dart b/packages/flutter_tools/bin/fuchsia_tester.dart
index 10d8aca..b77017d 100644
--- a/packages/flutter_tools/bin/fuchsia_tester.dart
+++ b/packages/flutter_tools/bin/fuchsia_tester.dart
@@ -3,12 +3,14 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:math' as math;
 
 import 'package:args/args.dart';
 import 'package:flutter_tools/src/base/common.dart';
 import 'package:flutter_tools/src/base/context.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/io.dart';
+import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/context_runner.dart';
 import 'package:flutter_tools/src/dart/package_map.dart';
@@ -121,6 +123,7 @@
       enableObservatory: collector != null,
       previewDart2: true,
       precompiledDillPath: dillFile.path,
+      concurrency: math.max(1, platform.numberOfProcessors - 2),
     );
 
     if (collector != null) {