[flutter] Make it possible to build and run dart widget tests again

Looks like we lost the ability to run these when they were
moved from //topaz.

Test: fx set core.x64 --with=//src/experiences:dart_unittests && \
      fx build && \
      fx run-host-tests dart_widget_tests
Change-Id: Ib8e7e592dc62039cb2e0142647c3fc0fe1639184
Reviewed-on: https://fuchsia-review.googlesource.com/c/experiences/+/399798
Reviewed-by: Jason Campbell <jasoncampbell@google.com>
Commit-Queue: David Reveman <reveman@google.com>
diff --git a/settings/BUILD.gn b/settings/BUILD.gn
index d3214d1..3d6d31b 100644
--- a/settings/BUILD.gn
+++ b/settings/BUILD.gn
@@ -27,6 +27,7 @@
   if (host_os != "mac") {
     _flutter_tester_tests += [
       "//src/experiences/settings/bin:settings_tests($host_toolchain)",
+      "//src/experiences/settings/lib/widgets:dart_widget_tests($host_toolchain)",
     ]
   }