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

pkg/testing/lib/src/test_dart.dart

Index: pkg/testing/lib/src/test_dart.dart
diff --git a/pkg/testing/lib/src/test_dart.dart b/pkg/testing/lib/src/test_dart.dart
index 8c89ed5fa9a097e9f70a495c2583c1dc5fb042b9..9b6ce7eb54d81ca5726c896d0a2ef9c7fa18c4a9 100644
--- a/pkg/testing/lib/src/test_dart.dart
+++ b/pkg/testing/lib/src/test_dart.dart
@@ -29,8 +29,7 @@ class TestDart extends Suite {
 
   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);
   }