Our microbenchmarks now run on real devices.

Wire up ios-profile and ios-release. Switching between debug, profile and release starts the build process from scratch. (#4063)

We now match the spec much better, including handling dark theme.

The main thing we're still missing is the landscape layout.

Fixes #989

Fixes the breakage that https://github.com/flutter/flutter/pull/4092 would cause.

Small fixes to contributing documentation.

 - Using widgets/spinning_square.dart instead of non-existing
  examples/widgets for the `run -t` example
 - travis/test.sh -> dev/bots/test.sh
 - Couple of style changes.

For developers with the platform tools installed but no SDK, it prevents
crashing in the flutter tool in favor of more user-friendly error messages
downstream.

* New Material Design calculator example. (Attempt 2 after git fetch upstream).

* Responded to code review.

* Second round of code review.

Rev the dart sdk to 1.17.0-dev.4.1

Keeps up with the Joneses, pulling in the latest dev build (and with it a number of stability and perf improvements).

Looks like our intent here was to show the tab label rather than
"Instance of '_Page'".

Also, fix an issue where the scrollableKey wasn't being forwarded to the
actual scrollable.

Fixes #3090

Fix analyzer field overrides.

Cleans up 3 of the 4 violations of the `overriden_field` lint.

The last one is more interesting and I'll defer to someone closer to that code:

    [lint] Do not override fields. (packages/flutter/test/rendering/rendering_tester.dart, line 91, col 14)

@chinmaygarde

* rebuild when the build type changes

* handle local engine builds

* also check the timestamp

Cleanup dangling comment references.