Make test.dart run web tests on Windows.

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
===================================================================
--- tools/testing/dart/test_runner.dart	(revision 3505)
+++ tools/testing/dart/test_runner.dart	(working copy)
@@ -243,6 +243,12 @@
   void runCommand(String executable,
                   List<String> arguments,
                   void exitHandler(int exitCode)) {
+    if (new Platform().operatingSystem() == 'windows') {
+      // Windows can't handle the first command if it is a .bat file or the like
+      // with the slashes going the other direction.
+      // TODO(efortuna): Remove this when fixed (Issue 1306).
+      executable = executable.replaceAll('/', '\\');
+    }
     process = new Process.start(executable, arguments);
     process.exitHandler = exitHandler;
     startTime = new Date.now();