Fix path tests on windows.

utils/tests/pub/install/path/path_is_file_test.dart

Index: utils/tests/pub/install/path/path_is_file_test.dart
diff --git a/utils/tests/pub/install/path/path_is_file_test.dart b/utils/tests/pub/install/path/path_is_file_test.dart
index 56218367f98f7066ad1a68378d01817cb35693c6..8717b49e56080286e8115059b49ed38d7ec10313 100644
--- a/utils/tests/pub/install/path/path_is_file_test.dart
+++ b/utils/tests/pub/install/path/path_is_file_test.dart
@@ -27,9 +27,15 @@ main() {
       })
     ]).scheduleCreate();
 
+    // TODO(rnystrom): The "\" in a Windows path gets treated like a regex
+    // character, so hack escape. A better fix is to use a literal string
+    // instead of a RegExp to validate, but that requires us to move the
+    // stack traces out of the stderr when we invoke pub. See also: #4706.
+    var escapePath = dummyPath.replaceAll(r"\", r"\\");
+
     schedulePub(args: ['install'],
         error: new RegExp("Path dependency for package 'foo' must refer to a "
-                          "directory, not a file. Was '$dummyPath'."),
+                          "directory, not a file. Was '$escapePath'."),
         exitCode: exit_codes.DATA);
   });
 }