Switch from interfaces to abstract classes in dart:io.

tests/co19/test_config.dart

Index: tests/co19/test_config.dart
diff --git a/tests/co19/test_config.dart b/tests/co19/test_config.dart
index c904a2aa226acb991a42d10ddd2dd94aee261a7e..4dbcc303c6bb7f6962376bfb5b41b1f6a4c0e924 100644
--- a/tests/co19/test_config.dart
+++ b/tests/co19/test_config.dart
@@ -13,7 +13,7 @@ class Co19TestSuite extends StandardTestSuite {
   Co19TestSuite(Map configuration)
       : super(configuration,
               "co19",
-              const Path("tests/co19/src"),
+              new Path("tests/co19/src"),
               ["tests/co19/co19-compiler.status",
                "tests/co19/co19-runtime.status",
                "tests/co19/co19-dart2js.status"]);