Fix path tests on windows.

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

Index: utils/tests/pub/install/path/nonexistent_dir_test.dart
diff --git a/utils/tests/pub/install/path/nonexistent_dir_test.dart b/utils/tests/pub/install/path/nonexistent_dir_test.dart
index 324a37b3c532a8e64897b7dd4263017dec99e88c..4a129f465e36658498cbde604e031ae7320922d7 100644
--- a/utils/tests/pub/install/path/nonexistent_dir_test.dart
+++ b/utils/tests/pub/install/path/nonexistent_dir_test.dart
@@ -23,9 +23,15 @@ 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 = badPath.replaceAll(r"\", r"\\");
+
     schedulePub(args: ['install'],
         error:
-            new RegExp("Could not find package 'foo' at '$badPath'."),
+            new RegExp("Could not find package 'foo' at '$escapePath'."),
         exitCode: exit_codes.DATA);
   });
 }