Throw, if `BlockSyntax.parseLines` loops indefinitely (dart-lang/markdown#533)

We throw an `AssertionError` if `BlockSyntax.parseLines` enters an
infinite loop. This should have no effect, if there are no bugs.
But if there is a bug, it's easier to discover it in production if the
failure mode isn't an infinite loop eating up CPU, but just an unhandled
exception.