Add a test runner executable.

test/io.dart

Index: test/io.dart
diff --git a/test/io.dart b/test/io.dart
index 663d454c11da659dba0ddfd96e81db6b0429d216..dc0222a2d911cbbedd2afc7870bdd973b1103c03 100644
--- a/test/io.dart
+++ b/test/io.dart
@@ -17,9 +17,14 @@ String _computePackageDir() {
   return p.dirname(p.dirname(p.fromUri(trace.frames.first.uri)));
 }
 
-/// Returns a matcher that matches a [FileSystemException] with the given
-/// [message].
-Matcher isFileSystemException(String message) => predicate(
-    (error) => error is FileSystemException && error.message == message,
-    'is a FileSystemException with message "$message"');
+/// Runs the unittest executable with the package root set properly.
+ProcessResult runUnittest(List<String> args, {String workingDirectory}) {
+  var allArgs = Platform.executableArguments.toList()
+     ..add(p.join(packageDir, 'bin/unittest.dart'))
+     ..add("--package-root=${p.join(packageDir, 'packages')}")
+     ..addAll(args);
 
+  // TODO(nweiz): Use ScheduledProcess once it's compatible.
+  return Process.runSync(Platform.executable, allArgs,
+      workingDirectory: workingDirectory);
+}