Throw if trying to set fs.currentDirectory in tests (#22037)

* Block setting of fs.currentDirectory in tests

This isn't perfect, it only covers tests using testUsingContext, but that is the huge majority of tests.

* Tweak error message.

packages/flutter_tools/test/src/context.dart

@@ -81,6 +81,7 @@ void testUsingContext(String description, dynamic testMethod(), {
           SimControl: () => MockSimControl(),
           Usage: () => MockUsage(),
           XcodeProjectInterpreter: () => MockXcodeProjectInterpreter(),
+          FileSystem: () => LocalFileSystemBlockingSetCurrentDirectory(),
         },
         body: () {
           final String flutterRoot = getFlutterRoot();
@@ -300,3 +301,13 @@ class MockFlutterVersion extends Mock implements FlutterVersion {}
 class MockClock extends Mock implements Clock {}
 
 class MockHttpClient extends Mock implements HttpClient {}
+
+class LocalFileSystemBlockingSetCurrentDirectory extends LocalFileSystem {
+  @override
+  set currentDirectory(dynamic value) {
+    throw 'fs.currentDirectory should not be set on the local file system during '
+          'tests as this can cause race conditions with concurrent tests. '
+          'Consider using a MemoryFileSystem for testing if possible or refactor '
+          'code to not require setting fs.currentDirectory.';
+  }
+}