This is an attempt to reland #18488 with less breakage on macOS.

Our style guide says the k's are not necessary, and it seems like a good idea to make all the code be consistent on this.

Only naming changes to private vars: no logic changes.

* Search for a suitable ARM sysroot instead of hardcoding it;
* Add facility to explain why NDK was not found;

Very basic support for "flutter emulators" which just lists the available Android AVDs.

Relates to:

https://github.com/flutter/flutter/issues/14822
https://github.com/Dart-Code/Dart-Code/issues/490
https://github.com/flutter/flutter/issues/13379

Fixes https://github.com/flutter/flutter/issues/16332

This updates AppContext per the recommendations in #15352

Fixes #15352

* Revert "Revert "Add android license verification to doctor and some refactoring" (#14727)"

This reverts commit d2602947.

* Add tests, fix sdkManagerEnv and use it consistently, and rearrange Status object model

* AnsiSpinner needs to leave the cursor where it found it.

* fix tests

* Const constructor warning only shows up on windows...?

* Avoid crash if we can't find the home directory

* Make pathVarSeparator return a string in the mock

* Implement review comments

* Fix out-of-order problem on stop

Reports from that commit included:

* Doctor hanging
* Doctor prompting to install JDK
* AnsiStatus.cancel() printing elapsed time and extra newline
* Printing extra spinner character at the end of each line

* Initial version, seems to work

* Unit test for android license checker

* Cleanups

* Windows analyzer wants const.

* Refinements to timeout

* review comments

* Forgot a nit

flutter doctor --android-licenses requires Android sdkmanager version 26
or later. When sdkmanager is not available (SDKs earlier than v25),
direct users to SDK upgrade instructions. When it's installed but not
v26 or later, emit instructions to run sdkmanager --update.

Convenience getters for the the path to the Android SDK manager and the
currently installed version of the tool.

Pre-factoring to support better checks around the --android-licenses
command, which uses a feature of the SDK manager that is unsupported in
older versions of the tool.

* Add support for NDK discovery and add --prefer-shared-library option

We would like to be able to use native tools (e.g. simpleperf, gdb) with
precompiled flutter apps.  The native tools work much better with *.so
files instead of the custom formats the Dart VM uses by default.

The reason for using blobs / instruction snapshots is that we do not
want to force flutter users to install the Android NDK.

This CL adds a `--prefer-shared-library` flag to e.g. `flutter build
apk` which will use the NDK compiler (if available) to turn the
precompiled app assembly file to an `*.so` file.  If the NDK compiler is
not available it will default to the default behavior.

* Rebase, add test for NDK detection, augment flutter.gradle with @Input for flag

* Use InMemoryFileSystem for test

* Remove unused import

* Address some analyzer warnings

Previously, we were mapping certain named platforms
(e.g. `android-stable`) to their corresponding version.
this had two problems:

1. The version could become out of date. For instance, we had
   mapped `android-stable` to version 24, but the stable version
   is now 27.
2. The list of possible named versions wasn't comprehensive.
   Some Android SDKs just list the platform as `stable`, or
   `experimental`, etc.

This change updates the platform version detection to use
the `build.prop` file that exists in the platform directory
(only for cases where the version number is not encoded into
the directory name).

* round 2

* deal with null futures

* review

* review

* review

* review

Fixes https://github.com/flutter/flutter/issues/10703

Eliminates the need for the device/daemon code to get at the iOS/Android
tooling indirectly via Doctor. In tests, we now inject the workflow
objects (or mocks) directly.

* add min version checks for IntelliJ

* validate the installed versions of IntelliJ and the flutter plugin

* review comments

Fixes https://github.com/flutter/flutter/issues/9954

Add a flutter doctor --android-licenses command that locates and runs the Android SDK license manager (#9892)

See https://github.com/flutter/flutter/issues/8438

Fixes https://github.com/flutter/flutter/issues/8690

* upgrade to linter-0.1.30

* add prefer_is_empty lint
* add directives_ordering lint
* add no_adjacent_strings_in_list lint
* add no_duplicate_case_values lint
* add prefer_collection_literals lint
* add prefer_const_constructors lint
* add prefer_contains lint
* add prefer_initializing_formals lint
* add unnecessary_null_aware_assignments lint
* add unnecessary_null_in_if_null_operators lint

* Use Java bundled with Android Studio for gradle

* review comments

- [x] Add custom logic on MacOS to determine if Java is installed before invoking `java`.
- [x] Check JAVA_HOME, platform specific logic, and finally PATH to locate the `java` executable.
- [x] Improved doctor messages.

Fixes #8508
Fixes #8521

The JDK doesn't put `javac` in the PATH on Windows :(

Line was accidentally removed by #8320

Also, change the version check from `java` to `javac` since we need the JDK and not just the JRE.

This reverts commit 70d7fe3a.

The Java version missmatch on the bots has been resolved.

* Teach flutter tools to find gradle

Flutter tools will now use Gradle from Android Studio, which is now found automatically.

flutter doctor will verify that Android Studio has been installed, and that the included Gradle is at least version 2.14.1.

It is still possible to manually configure the path to Android Studio (flutter config --android-studio-dir=XXX) or Gradle (flutter config --gradle-dir=XXX), but this should only be necessary if they're installed somewhere non-standard.

Only tested on Linux and macOS for now.

Fixes #8131