Rewrite the DevTools CONTRIBUTING guide (#6585)

* Rewrite the DevTools CONTRIBUTING guide

* review comments

Author: kenzieschmoll

BETA_TESTING.md

@@ -42,7 +42,7 @@ git checkout master
 git reset --hard origin/master
 
 bash tool/update_flutter_sdk.sh
-bash tool/upgrade.sh
+devtools_tool pub-get --only-main --upgrade
 ```
 
 If some steps failed, remove the directory and redo to [Setup](#setup).

CONTRIBUTING.md

@@ -1,4 +1,4 @@
-# Coding agreements in DevTools
+# DevTools style guide
 
 We fully follow [Effective Dart](https://dart.dev/guides/language/effective-dart)
 and some items of

packages/README.md renamed to STYLE.md

@@ -0,0 +1,91 @@
+Testing for DevTools
+=======================
+
+DevTools is test covered by multiple types of tests, all of which are run on the CI for each DevTools PR / commit:
+
+1. Unit tests
+    - tests for business logic
+2. Widget tests
+    - tests for DevTools UI components using mock or fake data
+    - some widget tests may contain golden image testing
+3. Partial integration tests
+    - tests for DevTools UI and business logic with a real VM service connection to a test app
+4. Full integration tests
+    - Flutter web integration tests that run DevTools as a Flutter web app and connect it to a real test app
+    on multiple platforms (Flutter mobile, Flutter web, and Dart CLI)
+
+**The following instructions are for unit tests, widget tests, and partial integration tests** in DevTools. For instructions
+on running and writing full integration tests, please see [integration_test/README.md](integration_test/README.md).
+In general, we should first try to test cover new features and bug fixes with unit tests or widget tests
+before writing new integration tests, which are slower to run and are not as easy to debug or iterate upon.
+
+## Running DevTools tests
+
+Before running tests, make sure your Flutter SDK matches the version that will be used on
+the CI. To update your local flutter version, run:
+
+```
+./tool/update_flutter_sdk.sh --local
+```
+> Note: Running this command requires that you have followed the [set up instructions](CONTRIBUTING.md#set-up-your-devtools-environment)
+in the DevTools contributing guide regarding cloning the Flutter SDK from GitHub.
+
+Now you can proceed with running DevTools tests:
+
+```
+cd packages/devtools_app
+flutter test test/
+```
+
+### Updating golden image files
+
+> Note: golden images should only be generated on MacOS.
+
+Golden image tests will fail for one of three reasons:
+
+1. The UI has been _intentionally_ modified.
+2. Something changed in the Flutter framework that would cause downstream changes for our tests.
+3. The UI has been _unintentionally_ modified, in which case we should not accept the changes.
+
+For valid golden image updates (1 and 2 above), the failing golden images will need to be updated. This can
+be done in one of two ways:
+
+1. If the tests failed on the CI for a PR, we can download the generated golden images directly from GitHub.
+    > If you are developing on a non-MacOS machine, this is the only way you'll be able to update the golden images. 
+    - Natvigate to the failed Actions run for your PR on GitHub. Example:
+
+        ![Failed actions run](_markdown_images/failed_actions_run.png)
+    
+    - Scroll to the bottom of the Summary view to see the errors from the `macos goldens` job, and the notice containing the golden update command:
+
+        ![Failed goldens notice](_markdown_images/failed_goldens_notice.png)
+
+2. Update the goldens locally by running the failing test(s) with the `--update-goldens` flag.
+
+    - Before updating the goldens, ensure your version of Flutter matches the version of Flutter that is used
+    on the CI. To update your local flutter version, run:
+
+        ```
+        ./tool/update_flutter_sdk.sh --local
+        ```
+
+    - Then proceed with updating the goldens:
+
+        ```
+        flutter test <path/to/my/test> --update-goldens
+        ```
+
+        or to update goldens for all tests:
+        ```
+        flutter test test/ --update-goldens
+        ```
+
+## Writing DevTools tests
+
+When you add a new feature or fix a bug, please add a corresponding test for your change.
+
+- If there is an existing test file for the feature your code touches, you can add the test case
+there. 
+- Otherwise, create a new test file with the `_test.dart` suffix, and place it in an appropriate
+location under the `test/` directory for the DevTools package you are working on.
+