Change dart:io to use Future for one-shot operations.

tests/standalone/io/process_run_output_test.dart

Index: tests/standalone/io/process_run_output_test.dart
diff --git a/tests/standalone/io/process_run_output_test.dart b/tests/standalone/io/process_run_output_test.dart
index c0e1796cea07237df2b8c852b3caa63ce8d35367..4d8ae846b9d77a132a8446dbf5571ede6861b3f9 100644
--- a/tests/standalone/io/process_run_output_test.dart
+++ b/tests/standalone/io/process_run_output_test.dart
@@ -31,19 +31,21 @@ test(scriptFile, encoding, stream) {
   var options = new ProcessOptions();
   if (stream == 'stdout') {
     options.stdoutEncoding = enc;
-    new Process.run(new Options().executable, [scriptFile, encoding, stream],
-                    options, (exit, out, err) {
-      Expect.equals(exit, 0);
-      Expect.equals(err, '');
-      checkOutput(encoding, out);
+    Process.run(new Options().executable,
+                [scriptFile, encoding, stream],
+                options). then((result) {
+      Expect.equals(result.exitCode, 0);
+      Expect.equals(result.stderr, '');
+      checkOutput(encoding, result.stdout);
     });
   } else {
     options.stderrEncoding = enc;
-    new Process.run(new Options().executable, [scriptFile, encoding, stream],
-                    options, (exit, out, err) {
-      Expect.equals(exit, 0);
-      Expect.equals(out, '');
-      checkOutput(encoding, err);
+    Process.run(new Options().executable,
+                [scriptFile, encoding, stream],
+                options).then((result) {
+      Expect.equals(result.exitCode, 0);
+      Expect.equals(result.stdout, '');
+      checkOutput(encoding, result.stderr);
     });
   }
 }