Dart SDK is not configured

I had the same problem after importing a flutter project from git. These are the steps to solve the problem.

1. File->Settings->Language & Framework->Flutter
2. Choose flutter SDK path: the first time we install flutter, we choose the location where the flutter should be installed. Choose this location.
3. Click OK and the android studio will refresh. Carry on if the problem is solved.
4. If you are still stuck with the error.
5. Goto this link and install Dart.
6. Goto the same place in settings, ..Language & Framework->Dart and chose the SDK location.

This solved the issues for me.

It usually happens with projects that were created in other machines. To fix this on Android Studio 3.1.3:

1. File-> Settings (ctrl+alt+s)
2. Languages and Frameworks -> Dart
3. Check "Enable Dart support for the project..."
4. In "Dart SDK path" click in "..." and navigate to flutter SDK directory. Under that directory you'll find "bin/cache/dart-sdk". This is the dart sdk path you should use.
5. Click "Apply"
6. Close the project and open it again (sometimes you need this step, sometimes doesn't)

Edit 2019-05-28 - I don't know how long this option is enabled but I have noticed that in Android Studio 3.4 it's easier to Enable Dart Support in projects that were developed in other machines.

1. File -> Sync Project With Gradle Files
2. After it builds, click in "Enable dart support" in the top of editor panel.

I had the same Dart SDK issue and I used this for my solution. To remedy the issue without having to use the terminal to test my programs each time the steps I followed are as follows.

1. Find flutter SDK path. I did so by typing in echo $PATH and finding the path with /flutter/bin.
2. Assuming your path is something like .../flutter/bin just add on /cache/dart-sdk/bin making it .../flutter/bin/cache/dart-sdk/bin.
3. This .../flutter/bin/cache/dart-sdk/bin is the location of your Dart SDK. Assuming you say the same Dart SDK is not configured error accompanied by something where you can configure your Dart SDK. You click the configure option and set .../flutter/bin/cache/dart-sdk as your SDK. That should fix your problem. If not head over to the next step.
4. Open your Android Studio preferences(Command + ',') and go to Languages and Frameworks>Dart.
5. Under the Dart menu, you should be able to enter your Dart SDK path.

I went through these steps using IntelliJ IDEA so they should be pretty much the same. If that didn't work, sometimes making a dummy Dart project can get things working again. That fixed a similar issue for me.