[flutter_tool] Prevent accidental calls to io.exit when asserts are active in unit tests (#46210)

packages/flutter_tools/lib/src/base/io.dart

@@ -26,7 +26,7 @@
 /// increase the API surface that we have to test in Flutter tools, and the APIs
 /// in `dart:io` can sometimes be hard to use in tests.
 import 'dart:async';
-import 'dart:io' as io show exit, IOSink, Process, ProcessInfo, ProcessSignal,
+import 'dart:io' as io show exit, IOSink, Platform, Process, ProcessInfo, ProcessSignal,
     stderr, stdin, Stdin, StdinException, Stdout, stdout;
 
 import 'package:meta/meta.dart';
@@ -92,12 +92,26 @@ ExitFunction _exitFunction = _defaultExitFunction;
 
 /// Exits the process.
 ///
+/// Throws [StateError] if assertions are enabled and the dart:io exit
+/// is still active when called. This may indicate exit was called in
+/// a test without being configured correctly. This behavior can be
+/// removed by setting the `FLUTTER_TEST` environment
+/// variable to a non-null string.
+///
 /// This is analogous to the `exit` function in `dart:io`, except that this
 /// function may be set to a testing-friendly value by calling
 /// [setExitFunctionForTests] (and then restored to its default implementation
 /// with [restoreExitFunction]). The default implementation delegates to
 /// `dart:io`.
-ExitFunction get exit => _exitFunction;
+ExitFunction get exit {
+  assert(
+    _exitFunction != io.exit || io.Platform.environment['FLUTTER_TEST'] != null,
+    'io.exit was called with assertions active. If this is an integration test, '
+    'ensure that the environment variable FLUTTER_TEST is set '
+    'to a non-null String.',
+  );
+  return _exitFunction;
+}
 
 /// Sets the [exit] function to a function that throws an exception rather
 /// than exiting the process; this is intended for testing purposes.

packages/flutter_tools/test/general.shard/base/io_test.dart

@@ -68,6 +68,16 @@ void main() {
   testUsingContext('ProcessSignal toString() works', () async {
     expect(io.ProcessSignal.sigint.toString(), ProcessSignal.SIGINT.toString());
   });
+
+  test('exit throws a StateError if called without being overriden', () {
+    expect(() => exit(0), throwsA(isInstanceOf<AssertionError>()));
+  });
+
+  test('exit does not throw a StateError if overriden', () {
+    setExitFunctionForTests((int value) {});
+
+    expect(() => exit(0), returnsNormally);
+  });
 }
 
 class MockIoProcessSignal extends Mock implements io.ProcessSignal {}

packages/flutter_tools/test/integration.shard/daemon_mode_test.dart

@@ -29,6 +29,9 @@ void main() {
     final Process process = await processManager.start(
       <String>[flutterBin, '--show-test-device', 'daemon'],
       workingDirectory: tempDir.path,
+      environment: <String, String>{
+        'FLUTTER_TEST': 'true',
+      }
     );
 
     final StreamController<String> stdout = StreamController<String>.broadcast();

packages/flutter_tools/test/integration.shard/test_utils.dart

@@ -46,7 +46,12 @@ Future<void> getPackages(String folder) async {
     'pub',
     'get',
   ];
-  final ProcessResult result = await processManager.run(command, workingDirectory: folder);
+  final ProcessResult result = await processManager.run(command,
+    workingDirectory: folder,
+    environment: <String, String>{
+    'FLUTTER_TEST': 'true'
+    },
+  );
   if (result.exitCode != 0) {
     throw Exception('flutter pub get failed: ${result.stderr}\n${result.stdout}');
   }