[flutter_tools] Small addition to the readme about testing (#50051)

packages/flutter_tools/README.md

 # Flutter Tools
 
-Command line developer tools for building Flutter applications.
+This section of the Flutter repository contains the command line developer tools
+for building Flutter applications.
 
 ## Working on Flutter Tools
 
 Be sure to follow the instructions on [CONTRIBUTING.md](../../CONTRIBUTING.md)
-to set up your development environment.
+to set up your development environment. Further, familiarize yourself with the
+[style guide](https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo),
+which we follow.
 
-### Setup
+### Setting up
 
 First, ensure that the Dart SDK and other necessary artifacts are available by
 invoking the Flutter Tools wrapper script. In this directory run:
@@ -15,7 +18,7 @@ invoking the Flutter Tools wrapper script. In this directory run:
 $ ../../bin/flutter --version
 ```
 
-### Running
+### Running the Tool
 
 To run Flutter Tools from source, in this directory run:
 ```shell
@@ -24,14 +27,36 @@ $ ../../bin/cache/dart-sdk/bin/dart bin/flutter_tools.dart
 followed by command line arguments, as usual.
 
 
-### Analyzing
+### Running the analyzer
 
 To run the analyzer on Flutter Tools, in this directory run:
 ```shell
 $ ../../bin/flutter analyze
 ```
 
-### Testing
+### Writing tests
+
+As with other parts of the Flutter repository, all changes in behavior [must be
+tested](https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#write-test-find-bug).
+Tests live under the `test/` subdirectory.
+- Hermetic unit tests of tool internals go under `test/general.shard`.
+- Tests of tool commands go under `test/commands.shard`. Hermetic tests go under
+  its `hermetic/` subdirectory. Non-hermetic tests go under its `permeable`
+  sub-directory.
+- Integration tests (e.g. tests that run the tool in a subprocess) go under
+  `test/integration.shard`.
+
+In general, the tests for the code in a file called `file.dart` should go in a
+file called `file_test.dart` in the subdirectory that matches the behavior of
+the test.
+
+We measure [test coverage](https://codecov.io/gh/flutter/flutter) post-submit.
+A change that deletes code might decrease test coverage, however most changes
+that add new code should aim to increase coverage. In particular the coverage
+of the diff should be close to the average coverage, and should ideally be
+better.
+
+### Running the tests
 
 To run the tests in the `test/` directory, first ensure that there are no
 connected devices. Then, in this directory run:
@@ -59,7 +84,6 @@ $ ../../bin/cache/dart-sdk/bin/pub run build_runner test
 ```
 This is what we do in the continuous integration bots.
 
-
 ### Forcing snapshot regeneration
 
 To force the Flutter Tools snapshot to be regenerated, delete the following