Rewrite Local.xcconfig per build

In case the user changes the path of the Flutter project on disk, the Dart SDK or the Flutter repository itself, the old Xcode paths would be invalid. Now, we rewrite them when we build the project.

packages/flutter_tools/lib/src/ios/device_ios.dart

@@ -565,12 +565,16 @@ String _getIOSEngineRevision(ApplicationPackage app) {
 }
 
 Future<bool> _buildIOSXcodeProject(ApplicationPackage app, { bool buildForDevice }) async {
+  String flutterProjectPath = Directory.current.path;
+
   if (xcodeProjectRequiresUpdate()) {
     printTrace('Initializing the Xcode project.');
-    if ((await setupXcodeProjectHarness()) != 0) {
+    if ((await setupXcodeProjectHarness(flutterProjectPath)) != 0) {
       printError('Could not initialize the Xcode project.');
       return false;
     }
+  } else {
+    updateXcodeLocalProperties(flutterProjectPath);
   }
 
   if (!_validateEngineRevision(app))

packages/flutter_tools/lib/src/ios/setup_xcodeproj.dart

@@ -91,7 +91,7 @@ Future<bool> _inflateXcodeArchive(String directory, List<int> archiveBytes) asyn
   return true;
 }
 
-void _setupXcodeProjXcconfig(String filePath) {
+void updateXcodeLocalProperties(String projectPath) {
   StringBuffer localsBuffer = new StringBuffer();
 
   localsBuffer.writeln('// This is a generated file; do not edit or check into version control.');
@@ -106,7 +106,7 @@ void _setupXcodeProjXcconfig(String filePath) {
   String dartSDKPath = path.normalize(path.join(Platform.resolvedExecutable, '..', '..'));
   localsBuffer.writeln('DART_SDK_PATH=$dartSDKPath');
 
-  File localsFile = new File(filePath);
+  File localsFile = new File(path.join(projectPath, 'ios', '.generated', 'Local.xcconfig'));
   localsFile.createSync(recursive: true);
   localsFile.writeAsStringSync(localsBuffer.toString());
 }
@@ -130,9 +130,9 @@ bool xcodeProjectRequiresUpdate() {
   return false;
 }
 
-Future<int> setupXcodeProjectHarness() async {
+Future<int> setupXcodeProjectHarness(String flutterProjectPath) async {
   // Step 1: Fetch the archive from the cloud
-  String iosFilesPath = path.join(Directory.current.path, 'ios');
+  String iosFilesPath = path.join(flutterProjectPath, 'ios');
   String xcodeprojPath = path.join(iosFilesPath, '.generated');
   List<int> archiveBytes = await _fetchXcodeArchive();
 
@@ -149,7 +149,7 @@ Future<int> setupXcodeProjectHarness() async {
   }
 
   // Step 3: Populate the Local.xcconfig with project specific paths
-  _setupXcodeProjXcconfig(path.join(xcodeprojPath, 'Local.xcconfig'));
+  updateXcodeLocalProperties(flutterProjectPath);
 
   // Step 4: Write the REVISION file
   File revisionFile = new File(path.join(xcodeprojPath, 'REVISION'));