Document test.windows= et al. (#12)

Author: Hixie

README.md

@@ -91,7 +91,8 @@ these tests to fail, you have the following options:
    people responsible for the breaking test(s) (see the relevant .test
    files), help them fix their code, and update this repository to use
    the new version of their tests, in addition to the steps described
-   on the wiki.
+   on the wiki. You will also need to land your change in two parts,
+   so that people have time to migrate (a "soft-breaking" change).
 
 3. Remove the test in question. This is by far the least ideal
    solution. To go down this path, we must first establish that one of

registry/template.test

@@ -96,18 +96,31 @@ update=.
 # shell script from that same directory, and the "extra_tests.dart"
 # program in the "dev" directory, run via "dart".
 #
-# These commands must work identically on Windows, macOS, and Linux.
-# This generally means you want to limit yourself to simple commands
-# like `flutter test`. For anything more elaborate, consider creating
-# a "tests" shell script to run on macOS and Linux and a "tests.bat"
-# batch file to run on Windows (referencing it without the extension
-# here should mean that in runs the shell script on Unix systems and
-# the batch file on Windows, because of how Windows looks up
-# commands). On Windows, these commands are executed via `CMD.EXE /S
-# /C "command..."`.
+# Commands specified using "test=" must work identically on Windows,
+# macOS, and Linux. This generally means you want to limit yourself to
+# simple commands like `flutter test`. For anything more elaborate,
+# consider creating a "tests" shell script to run on macOS and Linux
+# and a "tests.bat" batch file to run on Windows (referencing it
+# without the extension here should mean that in runs the shell script
+# on Unix systems and the batch file on Windows, because of how
+# Windows looks up commands). On Windows, these commands are executed
+# via `CMD.EXE /S /C "command..."`.
+#
+# Alternatively, you can use a platform-specific test line:
+#   test.windows= to run a command on Windows only.
+#   test.macos= to run a command on MacOS only.
+#   test.linux= to run a command on Linux only.
+#   test.posix= to run a command on MacOS and Linux only.
+#
+# There must be sufficient test lines specified such that tests are
+# available on each of Windows, MacOS, and Linux. In the example
+# below, we run mytest.bat on Windows and mytest.sh on MacOS and Linux
+# (in both cases, from the "scripts" directory).
 test=flutter test
 test=more_tests
 test=dart dev/extra_tests.dart
+test.windows=scripts\mytest.bat
+test.posix=scripts/mytest.sh
 
 # To test your tests, check out the flutter/flutter repository and
 # then, from the root of that repository, run: