attach globals to state object on initialisation

[Rich-Harris]

Ref #908. Just leaving this failing test here for now. Not totally sure what the best approach to fixing it is.

[Rich-Harris]

Idea: any globals that are used in the template, we add them to the state object when the component is initialised (if they don't already exist). That way, we dispense with the need to do things like this on update:

var each_value = ('Object' in state ? state.Object : Object).keys(state.todos);

It means adding additional those properties to state, but I don't see that as a problem.

[Rich-Harris]

Turns out that approach makes a lot of sense. Unfortunately, it doesn't currently work for SSR, because generator.expectedProperties isn't populated, because that happens in generator.findDependencies, which... is only called by the DOM compiler.

Rather than hacking in a fix, I reckon it's probably time to deal with that design flaw — resolving all those dependencies is probably something that needs to happen in generator.parseJs. It'll mean a bit of refactoring.

[Rich-Harris]

Feeling pretty good about this. Generator now has a walkTemplate method which is invoked upon construction — all the dependency tracking happens at that point, rather than later in generator.contextualise. This means the DOM and SSR compilers are both going through the same process, and everything happens in an order that's somewhat easier to reason about.

Phase 2 of this refactor will probably be to move all the stuff that's currently happening in generator.contextualise into walkTemplate — i.e. the code transformation itself.