• I'm trying to add type definitions to a library.
• I can only change .d.ts files (not any of the .js files)
• The tests for the library use private properties that we don't want included in the published definition.
• But we want the test files to be run through the TypeScript compiler (hence the --checkJs and --allowJs in tsconfig.json)
• I'd like to include properties accessed for testing in a separate testing-only type declaration file (test/index.d.ts).

But in the current state, I get the following error:

$ tsc
test/test.js(4,7): error TS2339: Property '_retval' does not exist on type 'Foo'.

What can I change in the .d.ts files to get this to compile?