Replace the existing unittest APIs with the new runner infrastructure.

test/io.dart

Index: test/io.dart
diff --git a/test/io.dart b/test/io.dart
index 1dbc377eebf599728281be8e342a6657c3354061..b1632347bd9fb3f36d389a880abde1d0b8f9b842 100644
--- a/test/io.dart
+++ b/test/io.dart
@@ -18,10 +18,18 @@ String _computePackageDir() {
 
 /// 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=${p.join(packageDir, 'packages')}")
-     ..addAll(args);
+  var allArgs = [
+    p.join(packageDir, 'bin/unittest.dart'),
+    "--package-root=${p.join(packageDir, 'packages')}"
+  ]..addAll(args);
+
+  // TODO(nweiz): Use ScheduledProcess once it's compatible.
+  return runDart(allArgs, workingDirectory: workingDirectory);
+}
+
+/// Runs Dart.
+ProcessResult runDart(List<String> args, {String workingDirectory}) {
+  var allArgs = Platform.executableArguments.toList()..addAll(args);
 
   // TODO(nweiz): Use ScheduledProcess once it's compatible.
   return Process.runSync(Platform.executable, allArgs,