Scrollable dropdown, dropdown underline cosmetics - #4766

Changes since last roll:
```
9a98143 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (flutter#4767)
e74e8b3 [async] Update includes of async headers to new path (flutter#4760)
e2c4b27 Use Dart 2 camel case constants in the engine Dart libraries (flutter#4766)
9c1e484 Updates for Fuchsia roll. (flutter#4765)
14c940e Switch from fxl::Mutex to std::mutex (flutter#4764)
debf82c Roll Garnet (flutter#4759)
5bffdef Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (flutter#4761)
```

Changes since last roll:
```
b6df7a6 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (flutter#4771)
a6764db Add sources for Fuchsia target. (flutter#4763)
2d59006 [fuchsia] Remove unused header file. (flutter#4769)
9717063 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (flutter#4767)" (flutter#4768)
9a98143 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (flutter#4767)
e74e8b3 [async] Update includes of async headers to new path (flutter#4760)
e2c4b27 Use Dart 2 camel case constants in the engine Dart libraries (flutter#4766)
9c1e484 Updates for Fuchsia roll. (flutter#4765)
14c940e Switch from fxl::Mutex to std::mutex (flutter#4764)
debf82c Roll Garnet (flutter#4759)
5bffdef Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (flutter#4761)
```

* Roll engine to b6df7a6.

Changes since last roll:
```
b6df7a6 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (#4771)
a6764db Add sources for Fuchsia target. (#4763)
2d59006 [fuchsia] Remove unused header file. (#4769)
9717063 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)" (#4768)
9a98143 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)
e74e8b3 [async] Update includes of async headers to new path (#4760)
e2c4b27 Use Dart 2 camel case constants in the engine Dart libraries (#4766)
9c1e484 Updates for Fuchsia roll. (#4765)
14c940e Switch from fxl::Mutex to std::mutex (#4764)
debf82c Roll Garnet (#4759)
5bffdef Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (#4761)
```

…#4766)

* Roll engine to b6df7a6.

Changes since last roll:
```
b6df7a6 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (flutter#4771)
a6764db Add sources for Fuchsia target. (flutter#4763)
2d59006 [fuchsia] Remove unused header file. (flutter#4769)
9717063 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (flutter#4767)" (flutter#4768)
9a98143 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (flutter#4767)
e74e8b3 [async] Update includes of async headers to new path (flutter#4760)
e2c4b27 Use Dart 2 camel case constants in the engine Dart libraries (flutter#4766)
9c1e484 Updates for Fuchsia roll. (flutter#4765)
14c940e Switch from fxl::Mutex to std::mutex (flutter#4764)
debf82c Roll Garnet (flutter#4759)
5bffdef Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (flutter#4761)
```