Update "dart:" libraries to latest from Dart repo

[munificent]

DDC has its own copy of the "dart:" library sources and they were forked some time ago. Eventually, we want to unfork (#257), but, until, then we're going to rev our forked copy to be closer to up-to-date with the current version of Dart.

Here's what's left to do:

• dart:async
• dart:collection (in-progress)
  • (Looks like there are some changes in collection_patch to bring over.)
• dart:convert
• dart:core
  • HashMap and HashSet
  • StackTrace
• dart:isolate
• dart:math
• dart:typed_data

The web libraries are handled separately since they are actively maintained:

• dart:html
• dart:indexed_db
• dart:js
• dart:svg
• dart:web_audio
• dart:web_gl
• dart:web_sql

(cc @JacobR)

These are not supported by DDC:

• dart:developer
• dart:mirrors
• dart:io

[munificent]

cc @rakudrama @hterkelsen

[munificent]

I'll take StackTrace.

[jmesserly]

I'll take dart:math

[munificent]

Oh, sorry, Harry is working on dart:math. :)

[munificent]

I'll look at the dart:collection stuff.

[jmesserly]

have ya'll considered bringing over dart:io? We don't have it yet but dart2js provides some of it:
https://github.com/dart-lang/sdk/blob/c566fc30c617f149eac6b5f52ce8f072684f50c8/sdk/lib/_internal/js_runtime/lib/io_patch.dart

[jmesserly]

I'm going to follow up and see if I can snip the dart:io dependency, it might be we are including unused stuff in unittest module

[munificent]

I'll take a look at dart:isolate, though I'm guessing there's nothing there we want to bring over right now.

[jmesserly]

I think this was done, or at least the P1 stuff was. We need to unfork SDK though still.

[jmesserly]

(unforking SDK is covered by #257)