implement deferred loading in fasta

[sigmundch]

Tasks:

• generate the loadlibrary instruction when calling deferred.loadLibrary
• generate a top level method so represent a tearoff of loadLibrary (a method is needed to ensure that we preserve that two tearoffs are identical)
• determine deferred-access patterns of dependencies used within a method (e.g. if a constant is accessed via deferred prefix, dart2js needs to know so it can put the constant in a separate output unit).
  status: done, but can be improved to be more precise and efficient by recording the information while parsing.
• generate checks for deferred accesses.
• generate compile-time errors for uses of deferred types in API declarations and inheritance (requires deeper changes in the outline builder)

[dgrove]

@sigmundch any updates on this?

[sigmundch]

Just updated the comment above: we are now passing dart2js unit tests regarding correct splitting of deferred units. And I've validated that the output manually for a few small examples.

Next - I'll be validating them on a large app and move on to error checking.

[sigmundch]

quick update: runtime checks for using deferred elements before they are loaded are now inserted.

We are missing still compile-time errors (e.g. when extending a deferred class), which need to be handled likely in the outline builder stage. However, we reached the point where we have all we need to unblock the release of dart2js with kernel at this time, so I lowered the priority to P2 and unassigned myself for now. I may be able to help implement that last part if needed in the future, just wanted to make it clear that I'm not actively pursuing it at this moment.