Add dev-dependency directly on stack_trace package. (#11)

* Add dev-dependency directly on stack_trace package.

Imports the package, but relied on an indirect dependency to ensure that the package was available.

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 70764c0..b2048cf 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -15,3 +15,4 @@
 dev_dependencies:
   shelf: '^0.6.0'
   test: '^0.12.0'
+  stack_trace: '^1.9.2'