Stop writing legacy FLUTTER_ROOT/version file (by default?) (flutter#172793)

Towards flutter#171900.

I'll make a breaking change announcement before landing this PR.

We can keep the flag for a stable release (allow folks to write `flutter
config --no-enable-omit-legacy-version-file` if they still need the
file), or we can decide it's not worth it and just rely on the breaking
change announcement.

Author: matanlurey

packages/flutter_tools/lib/src/features.dart

@@ -199,16 +199,13 @@ const swiftPackageManager = Feature(
 /// Whether to continue writing the `{FLUTTER_ROOT}/version` legacy file.
 ///
 /// Tracking removal: <https://github.com/flutter/flutter/issues/171900>.
-const omitLegacyVersionFile = Feature(
+const omitLegacyVersionFile = Feature.fullyEnabled(
   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),
 );
 
 /// Whether desktop windowing is enabled.

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

@@ -417,7 +417,6 @@ void main() {
 
           // Verify the version files exist and have been repopulated after the fetch.
           expect(FlutterVersion.getVersionFile(fs, flutterRoot), exists); // flutter.version.json
-          expect(fs.file(fs.path.join(flutterRoot, 'version')), exists); // legacy
 
           expect(flutterVersion.channel, channel);
           expect(flutterVersion.repositoryUrl, flutterUpstreamUrl);
@@ -1056,7 +1055,12 @@ void main() {
       expect(legacyVersionFile.existsSync(), isTrue);
       expect(legacyVersionFile.readAsStringSync(), '1.2.3');
     },
-    overrides: <Type, Generator>{ProcessManager: () => processManager, Cache: () => cache},
+    overrides: <Type, Generator>{
+      ProcessManager: () => processManager,
+      Cache: () => cache,
+      // ignore: avoid_redundant_argument_values
+      FeatureFlags: () => TestFeatureFlags(isOmitLegacyVersionFileEnabled: false),
+    },
   );
 
   testUsingContext(
@@ -1128,7 +1132,6 @@ void main() {
       final Directory flutterRoot = fs.directory(fs.path.join('path', 'to', 'flutter'));
       final Directory cacheDir = flutterRoot.childDirectory('bin').childDirectory('cache')
         ..createSync(recursive: true);
-      final File legacyVersionFile = flutterRoot.childFile('version');
       final File versionFile = cacheDir.childFile('flutter.version.json')..writeAsStringSync('{');
 
       processManager.addCommands(<FakeCommand>[
@@ -1202,12 +1205,10 @@ void main() {
 
       // version file was deleted because it couldn't be parsed
       expect(versionFile.existsSync(), isFalse);
-      expect(legacyVersionFile.existsSync(), isFalse);
       // version file was written to disk
       flutterVersion.ensureVersionFile();
       expect(processManager, hasNoRemainingExpectations);
       expect(versionFile.existsSync(), isTrue);
-      expect(legacyVersionFile.existsSync(), isTrue);
     },
     overrides: <Type, Generator>{ProcessManager: () => processManager, Cache: () => cache},
   );