Imports for types of ctor arguments are elided although they are lowered into a static property

[tbosch]

Reproduction

aclass.ts:

export class AClass {}

main.ts:

import {AClass} from './aclass';
class MyService {
  constructor(aclass: AClass) {}
}

Tsickle options: transformDecorators: true

Output:
main.js

// no import for AClass here!
...
MyService.ctorParameters = function () { return [
  // although AClass is now in a value position
  { type: AClass, },
]; };

Root cause
TypeScript right now has now way of allowing a transformer to tell it to not elide a symbol that is only used in a type position (see microsoft/TypeScript#17516).

Workaround
Set emitDecoratorMetadata: true. This way, TypeScript won't elide the import as its own transformer would have moved it into a value position.

[tbosch]

Note that this workaround is already applied in G3 and everywhere the current version of tsickle is used. This issue mostly serves the purpose of documenting this problem.