private static <T> T partialProxy(Class<T> type, OverridableInvocationHandler<T> handler) {
    return Reflection.newProxy(type, handler);
  }
InvocationHandler that forwards every method to an original object, except methods where there
is an implementation in this class with the same signature. So for example in the subclass
{@link ElementsHandler} there is a method {@link ElementsHandler#getTypeElement(CharSequence)},
which means that a call of {@link Elements#getTypeElement(CharSequence)} on the proxy with this
invocation handler will end up calling that method, but a call of any of the other methods of
{@code Elements} will end up calling the method on the original object.
value/src/test/java/com/google/auto/value/processor/GeneratedDoesNotExistTest.java