Replace travis with cirrus (flutter#5899)

Using a custom container image, we reduce the test time from 5 minutes to 2 minutes as we no longer run gclient sync from zero.

In the future, we may be able to cache the ninja output in the image (which probably gets updated daily) so we no longer compile from zero.

.cirrus.yml

@@ -30,3 +30,19 @@ task:
         ./flutter/tools/gn --android --unoptimized
         ninja -C out/android_debug_unopt
 
+format_and_dart_test_task:
+  container:
+    image: gcr.io/flutter-cirrus/build-engine-image:0.2
+
+  env:
+    CIRRUS_WORKING_DIR: "/tmp/github_repo"
+
+  replace_engine_script: |
+    cd $ENGINE_PATH/src
+    rm -r flutter
+    cp $CIRRUS_WORKING_DIR -r ./flutter
+    gclient sync
+
+  format_script: cd $ENGINE_PATH/src/flutter && ./travis/format.sh
+  build_script: cd $ENGINE_PATH/src/flutter && ./travis/build.sh
+  test_script: cd $ENGINE_PATH/src/flutter && ./travis/test.sh