Escape "\" in regex to unbreak Windows bot.

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

Index: sdk/lib/_internal/pub/test/build/warns_on_assets_paths_test.dart
diff --git a/sdk/lib/_internal/pub/test/build/warns_on_assets_paths_test.dart b/sdk/lib/_internal/pub/test/build/warns_on_assets_paths_test.dart
index 67eb1b5925ae2f0a19faca1fe9a8b2ddeccf7bbb..90c69f0b9bcb3c4da7e1ef375b753705715a602f 100644
--- a/sdk/lib/_internal/pub/test/build/warns_on_assets_paths_test.dart
+++ b/sdk/lib/_internal/pub/test/build/warns_on_assets_paths_test.dart
@@ -9,9 +9,13 @@ import 'package:path/path.dart' as path;
 import '../descriptor.dart' as d;
 import '../test_pub.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("\\", "\\\\");

[nweiz, 2013/12/19 19:34:34]

We should have a regExpQuote function in utils (with a TODO to remove it when
issue 4706 is fixed).

[Bob Nystrom, 2013/12/19 23:31:12]

Since this is already in, I don't think it's worth the effort now. (And we
generally try to avoid needing to quote regex strings at runtime.)

[nweiz, 2013/12/19 23:36:05]

I'm uncomfortable with the idea of TBRing something and then saying "it's
already in so it's not worth changing". TBRs should undergo just as much code
review as other CLs.
This doesn't make sense to me. RegExps are useful for validating test output,
building them dynamically can often make tests more readable, and quoting is
necessary when including arbitrary strings in RegExps.

[Bob Nystrom, 2013/12/27 22:53:01]

Gah, you're exactly right. I apologize.
Okey dokey.

+  return new RegExp(
+      '^Warning: Pub reserves paths containing "assets" for using assets from '

[nweiz, 2013/12/19 19:34:34]

Use r"" strings for these.

[Bob Nystrom, 2013/12/19 23:31:12]

The second line is using interpolation, and I think it's good for the first line
to be consistent.

+      'packages\\. Please rename the path "$assetsPath"\\.\$');
+}
 
 main() {
   initConfig();