Fix a test crash with Dart 1.12.

test/io.dart

Index: test/io.dart
diff --git a/test/io.dart b/test/io.dart
index e4c3f7ac242cb9c48e3b35dff24828752d816d8d..7bb87336f7be32c8280a03b7e97e8a738ad9f195 100644
--- a/test/io.dart
+++ b/test/io.dart
@@ -52,7 +52,7 @@ ProcessResult runDart(List<String> args, {String workingDirectory,
     // we're going to run in a different working directory.
     if (!arg.startsWith("--package-root=")) return arg;
     return "--package-root=" +
-        p.absolute(arg.substring("--package-root=".length));
+        p.absolute(p.fromUri(arg.substring("--package-root=".length)));
   }).toList()..addAll(args);
 
   // TODO(nweiz): Use ScheduledProcess once it's compatible.