compiler: return "if-test and throw" when compiling 'assert' (beta-3.0)

[mortend]

'assert' statements were turned into method calls to a method that was removed in 992b515, resulting in compile errors.

'assert' is still used in a few places, and this commit turns them into "if-test and throw" in debug builds (noop in release builds) to fix the compile errors.

assert VALUE;

becomes when compiled:

if (IS_DEBUG_BUILD && !VALUE)
    throw new Uno.InvalidOperationException(
        "Assertion failed: VALUE"
    );