wasm: use console.assert in dyld script

This commit uses console.assert() instead of node-specific strict
assert in the dyld script, in order to make it runnable in the
browser. console.assert() only warns and doesn't crash upon assertion
failure, but this is fine; we can always trivially define a strict
assert function shall it be necessary when debugging, and there hasn't
been such an assertion failure seen in the wild for long enough.