minor adjustments for TypeScript 3.5
minor adjustments for TypeScript 3.5

The "EmitTransformers" type is meant to match a corresponding TS type,
so use the types from that type to be agnostic to TS version.

Also delete some indirection that produces a type error under TS 3.5.

alexeagle/tsickle,angular/tsickle,alexeagle/tsickle,alexeagle/tsickle,angular/tsickle