Add a missing stack_trace dependency. (#51)

diff --git a/pubspec.yaml b/pubspec.yaml
index 2a5ba6b..a2c6bcb 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -6,5 +6,7 @@
 homepage: https://github.com/dart-lang/matcher
 environment:
   sdk: '>=1.0.0 <2.0.0'
+dependencies:
+  stack_trace: '^1.2.0'
 dev_dependencies:
   test: '>=0.12.0 <0.13.0'