[dsherret]

Type checking still occurs for `deno test`, which is what's mostly used for development. `deno run` won't have it by default because usually you run programs that are already type checked, but if you want it you will be able to add the `--check` flag (ex. `deno run --check main.ts`).

There's a huge performance overhead to run type checking, so the changes are for it to only be run for development/bundling workflows and be opt-in otherwise.