Move browser to dev_dependencies

Moved browser to dev_dependencies because it wasn't used in lib code.

The reason I am doing this is that it caused a compatibility issue in latest dart (Dart SDK version 1.5.0-dev.1.1). Although browser version was set to any in xml2json, pub complained its version didn't compatible with angular.

** Warning: Application may fail to run since packages did not get installed.Try running pub get again. **
Pub upgrade failed, [1] Resolving dependencies... (5.3s)
Incompatible version constraints on browser:
- angular 0.11.0 depends on version >=0.10.0 <0.11.0
- xml2json 2.0.2 depends on version >=0.9.1 <0.10.0