Add flutter config --enable-omit-legacy-version-file (#171903)

Towards flutter/flutter#171900.

We can decide how quickly to move here, but a feature flag will make it
easier to try out/rollback.

Author: matanlurey

packages/flutter_tools/lib/src/features.dart

@@ -53,6 +53,11 @@ abstract class FeatureFlags {
   /// Whether Swift Package Manager dependency management is enabled.
   bool get isSwiftPackageManagerEnabled => false;
 
+  /// Whether to stop writing the `{FLUTTER_ROOT}/version` file.
+  ///
+  /// Tracking removal: <https://github.com/flutter/flutter/issues/171900>.
+  bool get isOmitLegacyVersionFileEnabled => false;
+
   /// Whether a particular feature is enabled for the current channel.
   ///
   /// Prefer using one of the specific getters above instead of this API.
@@ -71,6 +76,7 @@ abstract class FeatureFlags {
     cliAnimation,
     nativeAssets,
     swiftPackageManager,
+    omitLegacyVersionFile,
   ];
 
   /// All current Flutter feature flags that can be configured.
@@ -176,6 +182,21 @@ const Feature swiftPackageManager = Feature(
   stable: FeatureChannelSetting(available: true),
 );
 
+/// Whether to continue writing the `{FLUTTER_ROOT}/version` legacy file.
+///
+/// Tracking removal: <https://github.com/flutter/flutter/issues/171900>.
+const Feature omitLegacyVersionFile = Feature(
+  name: 'stops writing the legacy version file',
+  configSetting: 'omit-legacy-version-file',
+  extraHelpText:
+      'If set, the file {FLUTTER_ROOT}/version is no longer written as part of '
+      'the flutter tool execution; a newer file format has existed for some '
+      'time in {FLUTTER_ROOT}/bin/cache/flutter.version.json.',
+  master: FeatureChannelSetting(available: true),
+  beta: FeatureChannelSetting(available: true),
+  stable: FeatureChannelSetting(available: true),
+);
+
 /// A [Feature] is a process for conditionally enabling tool features.
 ///
 /// All settings are optional, and if not provided will generally default to

packages/flutter_tools/lib/src/flutter_features.dart

@@ -51,6 +51,9 @@ mixin FlutterFeatureFlagsIsEnabled implements FeatureFlags {
 
   @override
   bool get isSwiftPackageManagerEnabled => isEnabled(swiftPackageManager);
+
+  @override
+  bool get isOmitLegacyVersionFileEnabled => isEnabled(omitLegacyVersionFile);
 }
 
 interface class FlutterFeatureFlags extends FeatureFlags with FlutterFeatureFlagsIsEnabled {

packages/flutter_tools/lib/src/version.dart

@@ -14,6 +14,7 @@ import 'base/time.dart';
 import 'base/utils.dart';
 import 'cache.dart';
 import 'convert.dart';
+import 'features.dart';
 import 'globals.dart' as globals;
 
 /// The default version when a version could not be determined.
@@ -689,6 +690,10 @@ void _ensureLegacyVersionFile({
   required String flutterRoot,
   required String frameworkVersion,
 }) {
+  // TODO(matanlurey): https://github.com/flutter/flutter/issues/171900.
+  if (featureFlags.isOmitLegacyVersionFileEnabled) {
+    return;
+  }
   final File legacyVersionFile = fs.file(fs.path.join(flutterRoot, 'version'));
   if (!legacyVersionFile.existsSync()) {
     legacyVersionFile.writeAsStringSync(frameworkVersion);

packages/flutter_tools/test/general.shard/version_test.dart

@@ -12,13 +12,14 @@ import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/base/process.dart';
 import 'package:flutter_tools/src/base/time.dart';
 import 'package:flutter_tools/src/cache.dart';
+import 'package:flutter_tools/src/features.dart';
 import 'package:flutter_tools/src/version.dart';
 import 'package:test/fake.dart';
 
 import '../src/common.dart';
 import '../src/context.dart';
 import '../src/fake_process_manager.dart';
-import '../src/fakes.dart' show FakeFlutterVersion;
+import '../src/fakes.dart' show FakeFlutterVersion, TestFeatureFlags;
 
 final SystemClock _testClock = SystemClock.fixed(DateTime.utc(2015));
 final DateTime _stampUpToDate = _testClock.ago(
@@ -1187,6 +1188,54 @@ void main() {
     overrides: <Type, Generator>{ProcessManager: () => processManager, Cache: () => cache},
   );
 
+  testUsingContext(
+    'legacy version file is still supported',
+    () {
+      final MemoryFileSystem fs = MemoryFileSystem.test();
+      final Directory flutterRoot = fs.directory(fs.path.join('path', 'to', 'flutter'));
+      flutterRoot.childDirectory('bin').childDirectory('cache').createSync(recursive: true);
+      final File legacyVersionFile = flutterRoot.childFile('version');
+
+      final FlutterVersion flutterVersion = FlutterVersion(
+        clock: _testClock,
+        fs: fs,
+        flutterRoot: flutterRoot.path,
+      );
+      flutterVersion.ensureVersionFile();
+
+      expect(legacyVersionFile, exists);
+    },
+    overrides: <Type, Generator>{
+      ProcessManager: () => FakeProcessManager.any(),
+      // ignore: avoid_redundant_argument_values
+      FeatureFlags: () => TestFeatureFlags(isOmitLegacyVersionFileEnabled: false),
+    },
+  );
+
+  testUsingContext(
+    'legacy version file is no longer supported',
+    () {
+      final MemoryFileSystem fs = MemoryFileSystem.test();
+      final Directory flutterRoot = fs.directory(fs.path.join('path', 'to', 'flutter'));
+      flutterRoot.childDirectory('bin').childDirectory('cache').createSync(recursive: true);
+      final File legacyVersionFile = flutterRoot.childFile('version');
+
+      final FlutterVersion flutterVersion = FlutterVersion(
+        clock: _testClock,
+        fs: fs,
+        flutterRoot: flutterRoot.path,
+      );
+      flutterVersion.ensureVersionFile();
+
+      expect(legacyVersionFile, isNot(exists));
+    },
+    overrides: <Type, Generator>{
+      ProcessManager: () => FakeProcessManager.any(),
+      // ignore: avoid_redundant_argument_values
+      FeatureFlags: () => TestFeatureFlags(isOmitLegacyVersionFileEnabled: true),
+    },
+  );
+
   testUsingContext('GitTagVersion', () {
     const String hash = 'abcdef';
     GitTagVersion gitTagVersion;

packages/flutter_tools/test/src/fakes.dart

@@ -501,6 +501,7 @@ class TestFeatureFlags implements FeatureFlags {
     this.isCliAnimationEnabled = true,
     this.isNativeAssetsEnabled = false,
     this.isSwiftPackageManagerEnabled = false,
+    this.isOmitLegacyVersionFileEnabled = false,
   });
 
   @override
@@ -536,6 +537,9 @@ class TestFeatureFlags implements FeatureFlags {
   @override
   final bool isSwiftPackageManagerEnabled;
 
+  @override
+  final bool isOmitLegacyVersionFileEnabled;
+
   @override
   bool isEnabled(Feature feature) {
     return switch (feature) {
@@ -549,6 +553,7 @@ class TestFeatureFlags implements FeatureFlags {
       flutterCustomDevicesFeature => areCustomDevicesEnabled,
       cliAnimation => isCliAnimationEnabled,
       nativeAssets => isNativeAssetsEnabled,
+      omitLegacyVersionFile => isOmitLegacyVersionFileEnabled,
       _ => false,
     };
   }
@@ -566,6 +571,7 @@ class TestFeatureFlags implements FeatureFlags {
     cliAnimation,
     nativeAssets,
     swiftPackageManager,
+    omitLegacyVersionFile,
   ];
 
   @override