Compute default value of -j option based on number of processors.

pkg/testing/lib/src/test_dart.dart

 // Copyright (c) 2016, the Dart project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
 library testing.test_dart;
 
 import 'dart:convert' show
     JSON;
 
 import 'dart:io' show
@@ skipping 11 matching lines @@
   final List<String> commandLines;
 
   TestDart(String name, this.common, this.processes, this.commandLines)
       : super(name, "test_dart",
           // This suite doesn't know what it's status file is because test.dart
           // doesn't know.
           null);
 
   factory TestDart.fromJsonMap(Uri base, Map json, String name, String kind) {
     String common = json["common"] ?? "";
-    // TODO(ahe): Compute this value based on number of cores.
-    String processes = json["processes"] ?? "-j16";
+    String processes = json["processes"] ?? "-j${Platform.numberOfProcessors}";
     List<String> commandLines = json["command-lines"] ?? <String>[];
     return new TestDart(name, common, processes, commandLines);
   }
 
   void writeFirstImportOn(StringSink sink) {
     sink.writeln("import 'dart:io' as io;");
     sink.writeln(
         "import 'package:testing/src/stdio_process.dart' show StdioProcess;");
   }
 
@@ skipping 30 matching lines @@
     }
   }
 """);
   }
 
   String toString() {
     return "TestDart($name, ${JSON.encode(common)}, ${JSON.encode(processes)},"
         " ${JSON.encode(commandLines)})";
   }
 }