More AppContext cleanups (#7073)

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

diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index da726d5..f727f38 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -56,7 +56,7 @@
   }
 
   /// Returns [XCode] active in the current app context.
-  static XCode get instance => context[XCode] ?? (context[XCode] = new XCode());
+  static XCode get instance => context[XCode];
 
   bool get isInstalledAndMeetsVersionCheck => isInstalled && xcodeVersionSatisfactory;
 

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

diff --git a/packages/flutter_tools/lib/src/ios/simulators.dart b/packages/flutter_tools/lib/src/ios/simulators.dart
index 6d405c8..d0bae71 100644
--- a/packages/flutter_tools/lib/src/ios/simulators.dart
+++ b/packages/flutter_tools/lib/src/ios/simulators.dart
@@ -37,9 +37,7 @@
 
 class IOSSimulatorUtils {
   /// Returns [IOSSimulatorUtils] active in the current app context (i.e. zone).
-  static IOSSimulatorUtils get instance {
-    return context[IOSSimulatorUtils] ?? (context[IOSSimulatorUtils] = new IOSSimulatorUtils());
-  }
+  static IOSSimulatorUtils get instance => context[IOSSimulatorUtils];
 
   List<IOSSimulator> getAttachedDevices() {
     if (!XCode.instance.isInstalledAndMeetsVersionCheck)
@@ -54,7 +52,7 @@
 /// A wrapper around the `simctl` command line tool.
 class SimControl {
   /// Returns [SimControl] active in the current app context (i.e. zone).
-  static SimControl get instance => context[SimControl] ?? (context[SimControl] = new SimControl());
+  static SimControl get instance => context[SimControl];
 
   Future<bool> boot({ String deviceName }) async {
     if (_isAnyConnected())