Replace all `goog.asserts` calls with new custom asserting functions

as part of https://github.com/protocolbuffers/protobuf-javascript/pull/128 we are inadvertently disabling all `goog.asserts` calls (this is an intended side effect of ADVANCED_COMPILATION)

we should instead migrate to our own asserting functions that will survive compilation.  This will also further progress on https://github.com/protocolbuffers/protobuf-javascript/issues/135