Implement flutter test -j (#20493)

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 @@ Future<Null> run(List<String> args) async {
       enableObservatory: collector != null,
       previewDart2: true,
       precompiledDillPath: dillFile.path,
+      concurrency: math.max(1, platform.numberOfProcessors - 2),
     );
 
     if (collector != null) {

packages/flutter_tools/lib/src/commands/test.dart

@@ -3,9 +3,11 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:math' as math;
 
 import '../base/common.dart';
 import '../base/file_system.dart';
+import '../base/platform.dart';
 import '../cache.dart';
 import '../runner/flutter_command.dart';
 import '../test/coverage_collector.dart';
@@ -77,7 +79,12 @@ class TestCommand extends FlutterCommand {
         negatable: false,
         help: 'Whether matchesGoldenFile() calls within your test methods should\n'
               'update the golden files rather than test for an existing match.',
-      );
+      )
+      ..addOption('concurrency',
+        abbr: 'j',
+        defaultsTo: math.max<int>(1, platform.numberOfProcessors - 2).toString(),
+        help: 'The number of concurrent test processes to run.',
+        valueHelp: 'jobs');
   }
 
   @override
@@ -109,7 +116,15 @@ class TestCommand extends FlutterCommand {
     if (startPaused && files.length != 1) {
       throwToolExit(
         'When using --start-paused, you must specify a single test file to run.',
-          exitCode: 1);
+        exitCode: 1,
+      );
+    }
+
+    final int jobs = int.tryParse(argResults['concurrency']);
+    if (jobs == null || jobs <= 0 || !jobs.isFinite) {
+      throwToolExit(
+        'Could not parse -j/--concurrency argument. It must be an integer greater than zero.'
+      );
     }
 
     Directory workDir;
@@ -135,8 +150,7 @@ class TestCommand extends FlutterCommand {
 
     final bool machine = argResults['machine'];
     if (collector != null && machine) {
-      throwToolExit(
-          "The test command doesn't support --machine and coverage together");
+      throwToolExit("The test command doesn't support --machine and coverage together");
     }
 
     TestWatcher watcher;
@@ -161,6 +175,7 @@ class TestCommand extends FlutterCommand {
       previewDart2: argResults['preview-dart-2'],
       trackWidgetCreation: argResults['track-widget-creation'],
       updateGoldens: argResults['update-goldens'],
+      concurrency: jobs,
     );
 
     if (collector != null) {

packages/flutter_tools/lib/src/test/runner.dart

@@ -5,8 +5,8 @@
 import 'dart:async';
 
 import 'package:args/command_runner.dart';
-// ignore: implementation_imports
-import 'package:test/src/executable.dart' as test;
+import 'package:meta/meta.dart';
+import 'package:test/src/executable.dart' as test; // ignore: implementation_imports
 
 import '../artifacts.dart';
 import '../base/common.dart';
@@ -34,7 +34,8 @@ Future<int> runTests(
   bool trackWidgetCreation = false,
   bool updateGoldens = false,
   TestWatcher watcher,
-    }) async {
+  @required int concurrency,
+}) async {
   if (trackWidgetCreation && !previewDart2) {
     throw new UsageException(
       '--track-widget-creation is valid only when --preview-dart-2 is specified.',
@@ -54,13 +55,7 @@ Future<int> runTests(
     testArgs.addAll(<String>['-r', 'compact']);
   }
 
-  if (enableObservatory) { // (In particular, for collecting code coverage.)
-    // Turn on concurrency, but just barely. This is a trade-off between running
-    // too many tests such that they all time out, and too few tests such that
-    // the tests overall take too much time. The current number is empirically
-    // based on what our infrastructure can handle, which isn't ideal...
-    testArgs.add('--concurrency=2');
-  }
+  testArgs.add('--concurrency=$concurrency');
 
   for (String name in names) {
     testArgs..add('--name')..add(name);