Prototype or not?

compress.sh

java -jar compiler.jar \
    --externs extern.js \
    --compilation_level ADVANCED_OPTIMIZATIONS \
    --js=main.js \
    --js_output_file=out.js

extern.js

/**
 * @externs
 */

var A;

/**
 * @constructor
 */
A.B = function() {}

/**
 * @param a
 * @returns {A.B}
 */
A.b = function(a) {}

/** @type {number} */
A.B.C;

main.js

let test = A.b(3);

console.log(test.C);