Add build test for Flutter codelabs (#19579)

* Add build test for Flutter codelabs

* Fix indentation in build test script

* Correct filename for the codelabs build test run by travis

* Change if statement to elif

* Replace echo statements with set -x

* Add BSD-style license

* Add codelabs build test to Cirrus

* Remove codelabs_build_test from Travis.

* Remove codelabs_build_test from travis_script.sh

* Update documentation for codelabs_build_test.sh

* Update readme for build test

* Add link to open a new issue to MDC-Flutter

* Add phrase to address "if you think you really need to break this test, please escalate this"
diff --git a/.cirrus.yml b/.cirrus.yml
index f5a7233..ba45aec 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -61,6 +61,11 @@
       container:
         cpu: 4
         memory: 8G
+    - name: codelabs-build-test
+      env:
+        SHARD: codelabs-build-test
+      build_test_script: ./dev/bots/codelabs_build_test.sh
+
 
 task:
   windows_container: