Look up executable on PATH when given environment variables.

tests/standalone/io/process_path_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// Test that the executable is looked up on the user's PATH when spawning a
+// process.
+
+#import('dart:io');
+
+main() {
+  // Pick an app that we expect to be on the PATH that returns 0 when run with
+  // no arguments.
+  var executable = Platform.operatingSystem == 'windows' ? 'cmd.exe' : 'true';
+
+  var options = new ProcessOptions();
+  Process.run(executable, []).then((result) {
+    Expect.equals(0, result.exitCode);
+  });
+}