Allow FlutterVersion to be overridden in the context (#8801)

packages/flutter_tools/lib/src/commands/upgrade.dart

@@ -32,8 +32,8 @@ class UpgradeCommand extends FlutterCommand {
       throwToolExit('Unable to upgrade Flutter: no upstream repository configured.');
     }
 
-    final FlutterVersion version = new FlutterVersion(Cache.flutterRoot);
-    if (version.channel == 'alpha') {
+    final FlutterVersion flutterVersion = FlutterVersion.instance;
+    if (flutterVersion.channel == 'alpha') {
       // The alpha branch is deprecated. Rather than trying to pull the alpha
       // branch, we should switch upstream to master.
       printStatus('Switching to from alpha to master...');
@@ -66,7 +66,7 @@ class UpgradeCommand extends FlutterCommand {
     );
 
     printStatus('');
-    printStatus(FlutterVersion.getVersion(Cache.flutterRoot).toString());
+    printStatus(flutterVersion.toString());
 
     final String projRoot = findProjectRoot();
     if (projRoot != null) {

packages/flutter_tools/lib/src/doctor.dart

@@ -15,6 +15,7 @@ import 'base/file_system.dart';
 import 'base/io.dart';
 import 'base/platform.dart';
 import 'base/process_manager.dart';
+import 'cache.dart';
 import 'device.dart';
 import 'globals.dart';
 import 'ios/ios_workflow.dart';
@@ -228,9 +229,9 @@ class _FlutterValidator extends DoctorValidator {
     final List<ValidationMessage> messages = <ValidationMessage>[];
     final ValidationType valid = ValidationType.installed;
 
-    final FlutterVersion version = FlutterVersion.getVersion();
+    final FlutterVersion version = FlutterVersion.instance;
 
-    messages.add(new ValidationMessage('Flutter at ${version.flutterRoot}'));
+    messages.add(new ValidationMessage('Flutter at ${Cache.flutterRoot}'));
     messages.add(new ValidationMessage(
       'Framework revision ${version.frameworkRevisionShort} '
       '(${version.frameworkAge}), ${version.frameworkDate}'

packages/flutter_tools/lib/src/runner/flutter_command_runner.dart

@@ -254,7 +254,7 @@ class FlutterCommandRunner extends CommandRunner<Null> {
 
     if (globalResults['version']) {
       flutterUsage.sendCommand('version');
-      printStatus(FlutterVersion.getVersion(Cache.flutterRoot).toString());
+      printStatus(FlutterVersion.instance.toString());
       return;
     }
 

packages/flutter_tools/lib/src/version.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'base/context.dart';
 import 'base/io.dart';
 import 'base/process.dart';
 import 'base/process_manager.dart';
@@ -16,7 +17,7 @@ final Set<String> kKnownBranchNames = new Set<String>.from(<String>[
 ]);
 
 class FlutterVersion {
-  FlutterVersion(this.flutterRoot) {
+  FlutterVersion._() {
     _channel = _runGit('git rev-parse --abbrev-ref --symbolic @{u}');
 
     final int slash = _channel.indexOf('/');
@@ -32,8 +33,6 @@ class FlutterVersion {
     _frameworkAge = _runGit('git log -n 1 --pretty=format:%ar');
   }
 
-  final String flutterRoot;
-
   String _repositoryUrl;
   String get repositoryUrl => _repositoryUrl;
 
@@ -55,7 +54,7 @@ class FlutterVersion {
   String get engineRevision => Cache.engineRevision;
   String get engineRevisionShort => _shortGitRevision(engineRevision);
 
-  String _runGit(String command) => runSync(command.split(' '), workingDirectory: flutterRoot);
+  String _runGit(String command) => runSync(command.split(' '), workingDirectory: Cache.flutterRoot);
 
   @override
   String toString() {
@@ -77,9 +76,7 @@ class FlutterVersion {
     return _runSync(<String>['git', 'log', '-n', '1', '--pretty=format:%ad', '--date=format:%Y-%m-%d %H:%M:%S'], Cache.flutterRoot);
   }
 
-  static FlutterVersion getVersion([String flutterRoot]) {
-    return new FlutterVersion(flutterRoot != null ? flutterRoot : Cache.flutterRoot);
-  }
+  static FlutterVersion get instance => context.putIfAbsent(FlutterVersion, () => new FlutterVersion._());
 
   /// Return a short string for the version (`alpha/a76bc8e22b`).
   static String getVersionString({ bool whitelistBranchName: false }) {