Clean up null assumptions for Xcode and CocoaPods classes (#83442)

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

diff --git a/packages/flutter_tools/lib/src/ios/xcode_build_settings.dart b/packages/flutter_tools/lib/src/ios/xcode_build_settings.dart
index f92eab3..f35aa8d 100644
--- a/packages/flutter_tools/lib/src/ios/xcode_build_settings.dart
+++ b/packages/flutter_tools/lib/src/ios/xcode_build_settings.dart
@@ -109,7 +109,7 @@
 /// Build name parsed and validated from build info and manifest. Used for CFBundleShortVersionString.
 String parsedBuildName({
   @required FlutterManifest manifest,
-  @required BuildInfo buildInfo,
+  BuildInfo buildInfo,
 }) {
   final String buildNameToParse = buildInfo?.buildName ?? manifest.buildName;
   return validatedBuildNameForPlatform(TargetPlatform.ios, buildNameToParse, globals.logger);
@@ -118,7 +118,7 @@
 /// Build number parsed and validated from build info and manifest. Used for CFBundleVersion.
 String parsedBuildNumber({
   @required FlutterManifest manifest,
-  @required BuildInfo buildInfo,
+  BuildInfo buildInfo,
 }) {
   String buildNumberToParse = buildInfo?.buildNumber ?? manifest.buildNumber;
   final String buildNumber = validatedBuildNumberForPlatform(
@@ -173,8 +173,9 @@
   final String buildNumber = parsedBuildNumber(manifest: project.manifest, buildInfo: buildInfo) ?? '1';
   xcodeBuildSettings.add('FLUTTER_BUILD_NUMBER=$buildNumber');
 
-  if (globals.artifacts is LocalEngineArtifacts) {
-    final LocalEngineArtifacts localEngineArtifacts = globals.artifacts as LocalEngineArtifacts;
+  final Artifacts artifacts = globals.artifacts;
+  if (artifacts is LocalEngineArtifacts) {
+    final LocalEngineArtifacts localEngineArtifacts = artifacts;
     final String engineOutPath = localEngineArtifacts.engineOutPath;
     xcodeBuildSettings.add('FLUTTER_ENGINE=${globals.fs.path.dirname(globals.fs.path.dirname(engineOutPath))}');
 

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

diff --git a/packages/flutter_tools/lib/src/ios/xcodeproj.dart b/packages/flutter_tools/lib/src/ios/xcodeproj.dart
index dbe3bd2..481fbd7 100644
--- a/packages/flutter_tools/lib/src/ios/xcodeproj.dart
+++ b/packages/flutter_tools/lib/src/ios/xcodeproj.dart
@@ -389,7 +389,7 @@
   }
   /// Returns unique scheme matching [buildInfo], or null, if there is no unique
   /// best match.
-  String? schemeFor(BuildInfo buildInfo) {
+  String? schemeFor(BuildInfo? buildInfo) {
     final String expectedScheme = expectedSchemeFor(buildInfo);
     if (schemes.contains(expectedScheme)) {
       return expectedScheme;