fix(client): Make the constructor a normal function in FlowEventMetadata
fix(client): Make the constructor a normal function in FlowEventMetadata

Without the `: function(`, and without babel, Firefox interprets the
`constructor` as a JS class constructor instead of a normal function,
which causes a blowup saying FlowEventMetadata is not a constructor.
Babel transpiles that difference away. I often run without babel to
make debugging simpler in Firefox.

This fixes that problem.

https://github.com/mozilla/fxa-content-server/pull/4390

r=philbooth
mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server