core/src/com/google/inject/internal/RealMultibinder.java
Key<T> getKeyForNewItem() {
    checkConfiguration(!bindingSelection.isInitialized(), "Multibinder was already initialized");
    return Key.get(
        bindingSelection.getElementTypeLiteral(),
        new RealElement(bindingSelection.getSetName(), MULTIBINDER, ""));
  }
Key<T> getKeyForNewItem() {
    checkConfiguration(!bindingSelection.isInitialized(), "Multibinder was already initialized");
    return Key.get(
        bindingSelection.getElementTypeLiteral(),
        new RealElement(bindingSelection.getSetName(), MULTIBINDER, ""));
  }
Adds a new entry to the set and returns the key for it.