Fix a crash (#7597) when the flutter sdk moves after getting packages (#7601)

* Add a check in case the flutter directory in .packages no longer exists. Clean up and prompt user

* Update documentation to use flutter packages get for end-users instead of flutter update-packages.

* Merge missing sdk error with the multiple sdk error. They're really the same thing.

* Use flutterPath in both checks.

* Change file_system’s copy folder to copy director which takes into account the file system

* Test support files

* Add test and split into 2 messages again.

* Move tests to run in memory file system's copy. Tested with dev/bots/test.sh

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

@@ -352,14 +352,28 @@ class FlutterCommandRunner extends CommandRunner<Null> {
         Uri rootUri = flutterUri.resolve('../../..');
         String flutterPath = path.normalize(fs.file(rootUri).absolute.path);
 
-        if (!_compareResolvedPaths(flutterPath, Cache.flutterRoot)) {
+        if (!fs.isDirectorySync(flutterPath)) {
           printError(
-            'Warning: the \'flutter\' tool you are currently running is different from the one referenced in your pubspec.yaml:\n'
-            '  running Flutter  : ${Cache.flutterRoot}\n'
-            '  pubspec reference: $flutterPath\n'
-            'This can happen when you have multiple copies of flutter installed. Please check your system path to verify\n'
-            'that you\'re running the expected version (run \'flutter --version\' to see which flutter is on your path). You\n'
-            'can also change which flutter your project points to by editing the \'flutter:\' path in your pubspec.yaml file.\n'
+            'Warning! This package referenced a Flutter repository via the .packages file that is\n'
+            'no longer available. The repository from which the \'flutter\' tool is currently\n'
+            'executing will be used instead.\n'
+            '  running Flutter tool: ${Cache.flutterRoot}\n'
+            '  previous reference  : $flutterPath\n'
+            'This can happen if you deleted or moved your copy of the Flutter repository, or\n'
+            'if it was on a volume that is no longer mounted or has been mounted at a\n'
+            'different location. Please check your system path to verify that you are running\n'
+            'the expected version (run \'flutter --version\' to see which flutter is on your path).\n'
+          );
+        } else if (!_compareResolvedPaths(flutterPath, Cache.flutterRoot)) {
+          printError(
+            'Warning! The \'flutter\' tool you are currently running is from a different Flutter\n'
+            'repository than the one last used by this package. The repository from which the\n'
+            '\'flutter\' tool is currently executing will be used instead.\n'
+            '  running Flutter tool: ${Cache.flutterRoot}\n'
+            '  previous reference  : $flutterPath\n'
+            'This can happen when you have multiple copies of flutter installed. Please check\n'
+            'your system path to verify that you are running the expected version (run\n'
+            '\'flutter --version\' to see which flutter is on your path).\n'
           );
         }
       }

packages/flutter_tools/test/data/dart_dependencies_test/changed_sdk_location/.packages

+flutter:file:///a/wild/non-existent/directory/has/appeared
+sdk-move-test:lib/

packages/flutter_tools/test/data/dart_dependencies_test/changed_sdk_location/lib/main.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// No content

packages/flutter_tools/test/data/dart_dependencies_test/changed_sdk_location/pubspec.yaml

+name: sdk-move-test
+
+dependencies:
+  flutter:
+    sdk: flutter

packages/flutter_tools/test/dependency_checker_test.dart

@@ -2,7 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:file/memory.dart';
+import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/platform.dart';
+import 'package:flutter_tools/src/cache.dart';
+import 'package:flutter_tools/src/commands/devices.dart';
 import 'package:flutter_tools/src/dart/dependencies.dart';
 import 'package:flutter_tools/src/dependency_checker.dart';
 import 'package:path/path.dart' as path;
@@ -14,6 +18,13 @@ void main()  {
   group('DependencyChecker', () {
     final String basePath = path.dirname(platform.script.path);
     final String dataPath = path.join(basePath, 'data', 'dart_dependencies_test');
+    MemoryFileSystem testFileSystem;
+
+    setUp(() {
+      Cache.disableLocking();
+      testFileSystem = new MemoryFileSystem();
+    });
+
     testUsingContext('good', () {
       final String testPath = path.join(dataPath, 'good');
       final String mainPath = path.join(testPath, 'main.dart');
@@ -67,5 +78,24 @@ void main()  {
       // the .dart file.
       expect(dependencyChecker.check(baseTime), isTrue);
     });
+
+    /// Test a flutter tool move.
+    ///
+    /// Tests that the flutter tool doesn't crash and displays a warning when its own location
+    /// changed since it was last referenced to in a package's .packages file.
+    testUsingContext('moved flutter sdk', () async {
+      String destinationPath = '/some/test/location';
+      // Copy the golden input and let the test run in an isolated temporary in-memory file system.
+      copyDirectorySync(
+        new LocalFileSystem().directory(path.join(dataPath, 'changed_sdk_location')),
+        fs.directory(destinationPath));
+      fs.currentDirectory = destinationPath;
+
+      // Doesn't matter what commands we run. Arbitrarily list devices here.
+      await createTestCommandRunner(new DevicesCommand()).run(<String>['devices']);
+      expect(testLogger.errorText, contains('.packages'));
+    }, overrides: <Type, Generator>{
+      FileSystem: () => testFileSystem,
+    });
   });
 }

packages/flutter_tools/test/src/base/file_system_test.dart

@@ -25,7 +25,7 @@ void main() {
       String targetPath = '/some/non-existent/target';
       Directory targetDirectory = targetMemoryFs.directory(targetPath);
       copyDirectorySync(sourceDirectory, targetDirectory);
-      
+
       expect(targetDirectory.existsSync(), true);
       targetMemoryFs.currentDirectory = targetPath;
       expect(targetMemoryFs.directory('empty_directory').existsSync(), true);