Update engine and temporarily get rid of custom build directories in `flutter build ios`

bin/cache/engine.version

-893425d96046e413e643d5de380dfaa2cc5725ed
+ed3014b3d337d025393bd894ffa2897e05d43e91

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

@@ -52,20 +52,15 @@ class BuildIOSCommand extends FlutterCommand {
 
     printStatus('Building the application for $logTarget.');
 
-    Directory buildDir = new Directory(path.join('build', 'ios_$logTarget'));
-
-    if (!buildDir.existsSync())
-      await buildDir.create();
-
     bool result = await buildIOSXcodeProject(app,
-        buildForDevice: !forSimulator, buildDirectory: buildDir, codesign: shouldCodesign);
+        buildForDevice: !forSimulator, codesign: shouldCodesign);
 
     if (!result) {
       printError('Encountered error while building for $logTarget.');
       return 1;
     }
 
-    printStatus('Built in ${buildDir.path}.');
+    printStatus('Built in ios/.generated.');
 
     return 0;
   }

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

@@ -97,7 +97,7 @@ bool _xcodeVersionCheckValid(int major, int minor) {
 }
 
 Future<bool> buildIOSXcodeProject(ApplicationPackage app,
-    { bool buildForDevice, Directory buildDirectory, bool codesign: true }) async {
+    { bool buildForDevice, bool codesign: true }) async {
   String flutterProjectPath = Directory.current.path;
 
   if (xcodeProjectRequiresUpdate()) {
@@ -132,15 +132,6 @@ Future<bool> buildIOSXcodeProject(ApplicationPackage app,
     'ONLY_ACTIVE_ARCH=YES',
   ];
 
-  if (buildDirectory != null) {
-    if (!buildDirectory.existsSync()) {
-      printError('The specified build directory ${buildDirectory.path} does not exist');
-      return false;
-    }
-
-    commands.add('TARGET_BUILD_DIR=${buildDirectory.absolute.path}');
-  }
-
   if (buildForDevice) {
     commands.addAll(<String>['-sdk', 'iphoneos', '-arch', 'arm64']);
     if (!codesign) {