Ensure target platform is passed is always passed to assemble (#43862)

packages/flutter_tools/lib/src/bundle.dart

@@ -50,7 +50,7 @@ class BundleBuilder {
   /// The default `mainPath` is `lib/main.dart`.
   /// The default  `manifestPath` is `pubspec.yaml`
   Future<void> build({
-    TargetPlatform platform,
+    @required TargetPlatform platform,
     BuildMode buildMode,
     String mainPath,
     String manifestPath = defaultManifestPath,

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

@@ -5,6 +5,8 @@
 import 'dart:async';
 import 'dart:math' as math;
 
+import 'package:meta/meta.dart';
+
 import '../application_package.dart';
 import '../base/common.dart';
 import '../base/context.dart';
@@ -417,7 +419,7 @@ class IOSSimulator extends Device {
   }
 
   Future<void> _setupUpdatedApplicationBundle(covariant BuildableIOSApp app, BuildInfo buildInfo, String mainPath) async {
-    await _sideloadUpdatedAssetsForInstalledApplicationBundle(app, buildInfo, mainPath);
+    await sideloadUpdatedAssetsForInstalledApplicationBundle(buildInfo, mainPath);
 
     // Step 1: Build the Xcode project.
     // The build mode for the simulator is always debug.
@@ -448,9 +450,11 @@ class IOSSimulator extends Device {
     await SimControl.instance.install(id, fs.path.absolute(bundle.path));
   }
 
-  Future<void> _sideloadUpdatedAssetsForInstalledApplicationBundle(ApplicationPackage app, BuildInfo buildInfo, String mainPath) {
+  @visibleForTesting
+  Future<void> sideloadUpdatedAssetsForInstalledApplicationBundle(BuildInfo buildInfo, String mainPath) {
     // Run compiler to produce kernel file for the application.
     return BundleBuilder().build(
+      platform: TargetPlatform.ios,
       mainPath: mainPath,
       precompiledSnapshot: false,
       buildMode: buildInfo.mode,

packages/flutter_tools/test/general.shard/ios/simulators_test.dart

@@ -8,6 +8,8 @@ import 'dart:io' show ProcessResult, Process;
 import 'package:file/file.dart';
 import 'package:flutter_tools/src/build_info.dart';
 import 'package:file/memory.dart';
+import 'package:flutter_tools/src/build_system/build_system.dart';
+import 'package:flutter_tools/src/build_system/targets/dart.dart';
 import 'package:flutter_tools/src/device.dart';
 import 'package:flutter_tools/src/application_package.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
@@ -183,6 +185,25 @@ void main() {
     });
   });
 
+  testUsingContext('builds with targetPlatform', () async {
+    final IOSSimulator simulator = IOSSimulator('x', name: 'iPhone X');
+    when(buildSystem.build(any, any)).thenAnswer((Invocation invocation) async {
+      return BuildResult(success: true);
+    });
+    await simulator.sideloadUpdatedAssetsForInstalledApplicationBundle(BuildInfo.debug, 'lib/main.dart');
+
+    final VerificationResult result = verify(buildSystem.build(any, captureAny));
+    final Environment environment = result.captured.single;
+    expect(environment.defines, <String, String>{
+      kTargetFile: 'lib/main.dart',
+      kTargetPlatform: 'ios',
+      kBuildMode: 'debug',
+      kTrackWidgetCreation: 'false',
+    });
+  }, overrides: <Type, Generator>{
+    BuildSystem: () => MockBuildSystem(),
+  });
+
   group('Simulator screenshot', () {
     MockXcode mockXcode;
     MockProcessManager mockProcessManager;
@@ -512,3 +533,5 @@ flutter:
     ProcessManager: () => FakeProcessManager.any(),
   });
 }
+
+class MockBuildSystem extends Mock implements BuildSystem {}