Bump dev dependencies on build packages (#72)

* Bump dev dependencies on build packages

Fixes Travis build error.

* Update CHANGELOG and pubspec.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index fb9e89d..a216870 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.22.1+4-dev
+
+* Bump and widen dev dependencies on build packages.
+
 ## 0.22.1+3
 
 Update implementations of the `cast()` and the deprecated `retype()` methods.
diff --git a/pubspec.yaml b/pubspec.yaml
index 593b4a7..fd5e16d 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: observable
-version: 0.22.1+3
+version: 0.22.1+4-dev
 author: Dart Team <misc@dartlang.org>
 description: Support for marking objects as observable
 homepage: https://github.com/dart-lang/observable
@@ -12,7 +12,7 @@
   quiver: '>=0.24.0 <0.30.0'
 dev_dependencies:
   dart_style: '^1.0.9'
-  build_runner: ^0.7.11
+  build_runner: '>=0.8.0 <0.10.0'
   build_test: ^0.10.0
-  build_web_compilers: ^0.3.1
+  build_web_compilers: '>=0.3.1 <0.5.0'
   test: ^0.12.0