DART

Running Gradle task assembleDebug....

Here is solution in my case.

Open your flutter Project directory.
Change directory to android directory in your flutter project directory cd android
clean gradle ./gradlew clean
Build gradle ./gradlew build or you can combine both commands with just ./gradlew clean build 
Now run your flutter project. 

Gradle task assembleDebug failed with exit code 1

delete .gradle folder in your flutter app (located in ~/android)
try run or debug again 
this should fix the build issues