Plumb strong mode option through startApp(...) to the engine on iOS. (#14653)

This is a temporary workaround until we solve #14594.

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

@@ -189,9 +189,14 @@ class IOSDevice extends Device {
       return new LaunchResult.failed();
     }
 
+    final bool strongMode = platformArgs['strong'] ?? false;
+
     // Step 3: Attempt to install the application on the device.
     final List<String> launchArguments = <String>['--enable-dart-profiling'];
 
+    if (strongMode)
+      launchArguments.add('--strong');
+
     if (debuggingOptions.startPaused)
       launchArguments.add('--start-paused');
 

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

@@ -316,9 +316,14 @@ class IOSSimulator extends Device {
         return new LaunchResult.failed();
     }
 
+    final bool strongMode = platformArgs['strong'] ?? false;
+
     // Prepare launch arguments.
     final List<String> args = <String>['--enable-dart-profiling'];
 
+    if (strongMode)
+      args.add('--strong');
+
     if (!prebuiltApplication) {
       args.addAll(<String>[
         '--flutter-assets-dir=${fs.path.absolute(getAssetBuildDirectory())}',