Add on_os and on_platform fields.

test/runner/configuration/configuration_test.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 file.
 
 @TestOn("vm")
 
 import 'package:boolean_selector/boolean_selector.dart';
 import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
 
@@ skipping 245 matching lines @@
 
       test("if the merge conflicts, prefers the new value", () {
         var older = new Configuration(
             timeout: new Timeout(new Duration(seconds: 1)));
         var newer = new Configuration(
             timeout: new Timeout(new Duration(seconds: 2)));
         var merged = older.merge(newer);
         expect(merged.timeout, equals(new Timeout(new Duration(seconds: 2))));
       });
     });
+
+    group("for onPlatform", () {
+      test("merges each nested configuration", () {
+        var merged = new Configuration(
+          onPlatform: {
+            new PlatformSelector.parse("vm"): new Configuration(verboseTrace: true),
+            new PlatformSelector.parse("chrome"): new Configuration(jsTrace: true)
+          }
+        ).merge(new Configuration(
+          onPlatform: {
+            new PlatformSelector.parse("chrome"): new Configuration(jsTrace: false),
+            new PlatformSelector.parse("firefox"): new Configuration(skip: true)
+          }
+        ));
+
+        expect(merged.onPlatform[new PlatformSelector.parse("vm")].verboseTrace,
+            isTrue);
+        expect(merged.onPlatform[new PlatformSelector.parse("chrome")].jsTrace,
+            isFalse);
+        expect(merged.onPlatform[new PlatformSelector.parse("firefox")].skip,
+            isTrue);
+      });
+    });
   });
 }