Escape "\" in regex to unbreak Windows bot.

sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart

Index: sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart b/sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart
index 467cf09cabeddc6ec9b90a2872253e1aedb57587..c4ae0c5dd1a53ac0370d2d621bb3ddd901fa15c5 100644
--- a/sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart
@@ -11,9 +11,13 @@ import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
-getWarningRegExp(String assetsPath) => new RegExp(
-    '^Warning: Pub reserves paths containing "assets" for using assets from '
-    'packages\\. Please rename the path "$assetsPath"\\.\$');
+getWarningRegExp(String assetsPath) {
+  // Escape backslashes since they are metacharacters in a regex.
+  assetsPath = assetsPath.replaceAll("\\", "\\\\");
+  return new RegExp(
+      '^Warning: Pub reserves paths containing "assets" for using assets from '
+      'packages\\. Please rename the path "$assetsPath"\\.\$');
+}
 
 main() {
   initConfig();