Flutter Android SDK Requirements

[Dan Field]

Flutter Master (and soon Dev) now requires Android SDK 28 and BuildTools version 28.0.3

This is a result of updating some functionality in the Engine and pushing up our target/compile SDK constraints to match.  

This came as a surprise to some users, and Gradle's error messages can be a bit tough to decipher here.  There's a patch open to update the Flutter Doctor output to help with this, and it should land soon.

If you're facing this issue, you should run:

Linux/Mac:

$ANDROID_HOME/tools/bin/sdkmanager "platforms;android-28" "build-tools;28.0.3"

Windows:

%ANDROID_HOME%\tools\bin\sdkmanager.bat "platforms;android-28" "build-tools;28.0.3"

Of course, you can install these components using the Android SDK manager via Android Studio as well.  Note that trying to run the above commands with Java version greater than 1.8 may result in errors; see https://stackoverflow.com/a/48704729/3293914 for a work around if your'e using later Java versions.

Please let me know if this causes problems, or feel free to comment on the following:

https://github.com/flutter/flutter/pull/26942 (Doctor pull request)

https://github.com/flutter/flutter/issues/26909 (issue)

Dan