* Roll engine to b6df7a637498ca9beda1fa9cd7210e3202ea599f.

Changes since last roll:
```
b6df7a637 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (#4771)
a6764dbd5 Add sources for Fuchsia target. (#4763)
2d5900615 [fuchsia] Remove unused header file. (#4769)
9717063b7 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)" (#4768)
9a9814312 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)
e74e8b35c [async] Update includes of async headers to new path (#4760)
e2c4b2760 Use Dart 2 camel case constants in the engine Dart libraries (#4766)
9c1e48434 Updates for Fuchsia roll. (#4765)
14c940e27 Switch from fxl::Mutex to std::mutex (#4764)
debf82c0b Roll Garnet (#4759)
5bffdefbb Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (#4761)
```

Code that uses dataPatterns[loc] expects it to contain Map<String, String>, not Map<String, dynamic>.

Change gen_date_localizations.dart to generate datePatterns with a more precise type and regenerate date_localizations.dart.

Small code changes as part of moving the framework SDK forward to a 2.0 dev version.

* remove locale "sd" (not supported by ICU/CLDR); unify localizations scripts CLI

* address comments

* upgrade intl data; use local numerals in date picker

* use the shiny new public intl API

* address comments

* merge with Hans; address comments

* merge material_localizations.dart