Add a function to pub for finding the canonical path of a file.

sdk/lib/_internal/pub/test/test_pub.dart

Index: sdk/lib/_internal/pub/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub/test/test_pub.dart b/sdk/lib/_internal/pub/test/test_pub.dart
index a175c7b89ac24d74956624fc9e9cc8f4ead04a4f..e2a06f58e3dd07a363e28c2fbfff5d4448a68b5a 100644
--- a/sdk/lib/_internal/pub/test/test_pub.dart
+++ b/sdk/lib/_internal/pub/test/test_pub.dart
@@ -355,7 +355,7 @@ ScheduledProcess startPub({List args, Future<Uri> tokenEndpoint}) {
   // If the executable looks like a path, get its full path. That way we
   // can still find it when we spawn it with a different working directory.
   if (dartBin.contains(Platform.pathSeparator)) {
-    dartBin = new File(dartBin).fullPathSync();
+    dartBin = path.absolute(dartBin);
   }
 
   // Find the main pub entrypoint.