VM: Function::KernelLibraryIndex() can return -1, causing KernelProgramInfo::KernelLibrary to be asked for the typed data for library -1

[jensjoha]

sdk/runtime/vm/object.cc

Lines 11424 to 11429 in 05fc6bd

	intptr_t Function::KernelLibraryIndex() const {
	ASSERT(!is_declared_in_bytecode());
	if (IsNoSuchMethodDispatcher() || IsInvokeFieldDispatcher() ||
	IsFfiCallbackTrampoline()) {
	return -1;
	}

sdk/runtime/vm/object.cc

Lines 15695 to 15701 in 05fc6bd

	TypedDataViewPtr KernelProgramInfo::KernelLibrary(
	intptr_t library_index) const {
	const intptr_t start_offset = KernelLibraryStartOffset(library_index);
	const intptr_t end_offset = KernelLibraryEndOffset(library_index);
	const auto& component = TypedDataBase::Handle(kernel_component());
	return component.ViewFromTo(start_offset, end_offset);
	}

This used "to work" because when reading the start and end offsets for the library, the read end was the start of library 0 and the read start was the nnbd component mode (a number between 0 and 4 I think). It then created a view from, say 0 to where-ever-lib-1-starts (and the data is then (hopefully) never really read (I think it only logically checks that the size is > 0)).
When removing the nnbd component mode it instead reads the "mainMethodReference" which is a semi-arbitrary number which can be larger than where-ever library 0 starts, the start can thus become larger than the end and debug builds become sad: https://dart-ci.firebaseapp.com/cl/416940/1

I'm adding some special handling in https://dart-review.googlesource.com/c/sdk/+/416940 but it should probably be fixed better so it never requests data for library -1.