Flutter fails on `flutter` command

For others who're facing a similar error, I suggest that you read the docs carefully. I didn't, hence I wasted quite some time looking out for the solution elsewhere, even though it was right in front of my eyes all the time: https://flutter.io/setup-macos/#update-your-path

I'm writing the commands down so that you can copy and paste them easily in your terminal window:

touch $HOME/.bash_profile
open $HOME/.bash_profile

Line 1 creates a text file named .bash_profile. Line 2 opens it.

Then the line given below has to be pasted in the text file:

export PATH=[PATH_TO_FLUTTER_GIT_DIRECTORY]/flutter/bin:$PATH

Don't forget to save the file after pasting. flutter doctor should work with every Terminal window now.

Looks like your Dart version is outdated < 1.9.2.
Try dart --version to get the version of your installed Dart SDK.
Current is 1.12.x.

Update 2018-03-20

During the migration phase to Dart 2 you might want to install Dart from the dev channel to get a 2.0.0-alpha.x version. The newest stable version is 1.24.x

For example

brew install dart --devel

Update 2018-06-21

Now also supported:

brew install dart@2