[flutter_tool] Add some useful commands to the README.md (#36560)

packages/flutter_tools/README.md

 # Flutter Tools
 
-Developer tools for building Flutter applications.
+Command line developer tools for building Flutter applications.
 
-Be sure to follow the instructions on
-[CONTRIBUTING.md](../../CONTRIBUTING.md) to setup.
+## Working on Flutter Tools
 
-To run the tests, ensure that no devices are connected,
-then navigate to `flutter_tools` and execute:
+Be sure to follow the instructions on [CONTRIBUTING.md](../../CONTRIBUTING.md)
+to set up your development environment.
 
+### Setup
+
+First, ensure that the Dart SDK and other necessary artifacts are available by
+invoking the Flutter Tools wrapper script. In this directory run:
+```shell
+$ ../../bin/flutter --version
+```
+
+### Running
+
+To run Flutter Tools from source, in this directory run:
+```shell
+$ ../../bin/cache/dart-sdk/bin/dart bin/flutter_tools.dart
+```
+followed by command line arguments, as usual.
+
+### Analyzing
+
+To run the analyzer on Flutter Tools, in this directory run:
+```shell
+$ ../../bin/flutter analyze
+```
+
+### Testing
+
+To run the tests in the `test/` directory, first ensure that there are no
+connected devices. Then, in this directory run:
+```shell
+$ ../../bin/cache/dart-sdk/bin/pub run test
+```
+
+The tests in `test/integration.shard` are slower to run than the tests in
+`test/general.shard`. To run only the tests in `test/general.shard`, in this
+directory run:
+```shell
+$ ../../bin/cache/dart-sdk/bin/pub run test test/general.shard
+```
+
+To run the tests in a specific file, run:
+```shell
+$ ../../bin/cache/dart-sdk/bin/pub run test test/general.shard/utils_test.dart
+```
+
+When running all of the tests, it is a bit faster to use `build_runner`. First,
+set `FLUTTER_ROOT` to the root of your Flutter checkout. Then, in this directory
+run:
+```shell
+$ ../../bin/cache/dart-sdk/bin/pub run build_runner test
+```
+
+### Forcing snapshot regeneration
+
+To force the Flutter Tools snapshot to be regenerated, delete the following
+files:
 ```shell
-../../bin/cache/dart-sdk/bin/pub run test
+$ rm ../../bin/cache/flutter_tools.stamp ../../bin/cache/flutter_tools.snapshot
 ```