Unify DDC SDK with Dart repo's SDK

[jmesserly]

DDC compiles against the Dart SDK that is installed, but runs against an older built snapshot from our implementation. Here's the plan to fix that:

• delete our 1.9 copy. Replace with latest.
• figure out how we store latest SDK. Ideally we don't check it in. Maybe DEPS or git submodules to point at a hash file?
• try to patch against our impls. It's an error if the patch files don't line up.
• fix the impls if needed (i.e. implement new APIs that are lacking DDC impl)
• save the JS file as well as DDC summary
• DDC must compile against its own SDK summary (otherwise there's no guarantee the impls will work)

thanks to @vsmenon for helping me brainstorm this :)

[munificent]

I'll take dart:async.

[vsmenon]

@jacob314 will look at dart:html once dart:async is landed

[munificent]

dart:async is done as of 8154650

[munificent]

Working on Uri and probably dart:convert (since Uri uses it) now.

[rakudrama]

Blocked by #253:
lib/collection/linked_list.dart
lib/collection/queue.dart
lib/collection/splay_map.dart

[munificent]

OK, Uri and dart:convert are landed, though @floitschG says there may be some more dart:convert changes coming soon after. What's left to do?

[munificent]

This issue was originally about unforking DDC's SDK but we've been using it to refer to rolling it. To disambiguate, I'm forking the bug. This one is now about unforking. The bug for just rolling the latest SDK is #565.

[jmesserly]

I investigated DDC summary. We can produce it with new Analyzers, but we can't build tests against it because some packages and a few tests seem to depend on dart:io (not sure if they use much of it or what's up there).

[jmesserly]

This issue was moved to dart-lang/sdk#27314