Update DEPS to check out co19 tests from dart.googlesource.com

Change-Id: I7b6b9e61a49ec9ef3ef80850893218590c208ca6
Reviewed-on: https://dart-review.googlesource.com/c/90010
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: William Hesse <whesse@google.com>
diff --git a/DEPS b/DEPS
index f44b9ec..0018973 100644
--- a/DEPS
+++ b/DEPS
@@ -177,7 +177,7 @@
   },
 
   Var("dart_root") + "/tests/co19_2/src":
-      Var("chromium_git") + "/external/github.com/dart-lang/co19.git" +
+      Var("dart_git") + "/co19.git" +
       "@" + Var("co19_2_rev"),
 
   Var("dart_root") + "/third_party/zlib":