Add deps for build_runner (#50)

Towards #48

Allows us to run `pub run build_runner test -- -p chrome` to test with
DDC. Exposes issues with the package and with tests so not enabling on
Travis for now.
diff --git a/pubspec.yaml b/pubspec.yaml
index 3c9467f..e57d3ef 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -11,3 +11,7 @@
   fake_async: "^0.1.2"
   stack_trace: "^1.0.0"
   test: "^0.12.0"
+  # For building and testing with DDC
+  build_runner: ^0.7.11
+  build_web_compilers: ^0.3.1
+  build_test: ^0.10.1