~/code/emscripten/tools/bindings_generator.py bindings ValueProvider.h -- '{ "create_js_implementations": [{ "class_name": "ConstValueProvider", "implements": ["ValueProvider"]}] }'

~/code/emscripten/em++ times2.cpp bindings.cpp -include bindings.all.h -o times2.js

Change ConstValueProvider.js to this:

function ConstValueProvider(x) {
  initObject(this, ConstValueProvider, _emscripten_construct_ConstValueProvider());

  this.x = x;
}
ConstValueProvider.prototype.getValue = function ConstValueProvider_getValue() {
  return this.x;
}

Click the button in times2.html.