Remove case expressions that was not compile time constants.
Remove case expressions that was not compile time constants.

Case expressions must be compile-time constants. This has been in the spec since 0.10.

R=sigmund@google.com
BUG=

Review URL: https://chromiumcodereview.appspot.com//23460011

taisph/dart-protoc-plugin,dart-lang/dart-protobuf,google/protobuf.dart,taisph/dart-protoc-plugin,dart-lang/dart-protobuf,dart-lang/dart-protobuf,taisph/dart-protoc-plugin,bwu-dart-contributing/dart-protoc-plugin,dart-lang/protobuf,google/protobuf.dart,google/protobuf.dart,dart-lang/protobuf,dart-lang/protobuf