Fix SwiftPM scheme migration to handle when there are no BuildActionEntries (#164660)

An Xcode scheme may or may not have `BuildActionEntries`. When migrating
to SwiftPM, if Flutter cannot find `BuildActionEntries` in the xcscheme,
append the `PreAction` to the end of the `BuildAction`.

Fixes flutter/flutter#163086.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: vashworth

packages/flutter_tools/lib/src/migrations/swift_package_manager_integration_migration.dart

@@ -311,18 +311,18 @@ class SwiftPackageManagerIntegrationMigration extends ProjectMigrator {
 $newContent
       </PreActions>
 ''';
-      final String? buildActionEntries =
+      String? buildAction =
           schemeLines.where((String line) => line.contains('<BuildActionEntries>')).firstOrNull;
-      if (buildActionEntries == null) {
-        throw Exception(
-          'Failed to parse ${schemeFile.basename}: Could not find BuildActionEntries.',
-        );
-      } else {
-        newScheme = schemeContent.replaceFirst(
-          buildActionEntries,
-          '$newContent$buildActionEntries',
-        );
+      if (buildAction == null) {
+        // If there are no BuildActionEntries, append before end of BuildAction.
+        buildAction =
+            schemeLines.where((String line) => line.contains('</BuildAction>')).firstOrNull;
+
+        if (buildAction == null) {
+          throw Exception('Failed to parse ${schemeFile.basename}: Could not find BuildAction.');
+        }
       }
+      newScheme = schemeContent.replaceFirst(buildAction, '$newContent$buildAction');
     }
 
     schemeFile.writeAsStringSync(newScheme);

packages/flutter_tools/test/general.shard/migrations/swift_package_manager_integration_migration_test.dart

@@ -464,7 +464,7 @@ void main() {
             },
           );
 
-          testWithoutContext('fails if cannot find BuildActionEntries in scheme', () async {
+          testWithoutContext('fails if cannot find BuildAction in scheme', () async {
             final MemoryFileSystem memoryFileSystem = MemoryFileSystem();
             final BufferLogger testLogger = BufferLogger.test();
             final FakeXcodeProject project = FakeXcodeProject(
@@ -490,7 +490,7 @@ void main() {
             await expectLater(
               () => projectMigration.migrate(),
               throwsToolExit(
-                message: 'Failed to parse Runner.xcscheme: Could not find BuildActionEntries',
+                message: 'Failed to parse Runner.xcscheme: Could not find BuildAction',
               ),
             );
           });
@@ -525,6 +525,42 @@ void main() {
             );
           });
 
+          testWithoutContext('successfully updates scheme with no BuildActionEntries', () async {
+            final MemoryFileSystem memoryFileSystem = MemoryFileSystem();
+            final BufferLogger testLogger = BufferLogger.test();
+            final FakeXcodeProject project = FakeXcodeProject(
+              platform: platform.name,
+              fileSystem: memoryFileSystem,
+              logger: testLogger,
+            );
+            _createProjectFiles(project, platform);
+            project.xcodeProjectSchemeFile().writeAsStringSync(
+              _validBuildActions(platform, hasBuildEntries: false),
+            );
+
+            final FakePlistParser plistParser = FakePlistParser.multiple(<String>[
+              _plutilOutput(_allSectionsMigratedAsJson(platform)),
+              _plutilOutput(_allSectionsMigratedAsJson(platform)),
+            ]);
+            final SwiftPackageManagerIntegrationMigration projectMigration =
+                SwiftPackageManagerIntegrationMigration(
+                  project,
+                  platform,
+                  BuildInfo.debug,
+                  xcodeProjectInterpreter: FakeXcodeProjectInterpreter(),
+                  logger: testLogger,
+                  fileSystem: memoryFileSystem,
+                  plistParser: plistParser,
+                  features: swiftPackageManagerFullyEnabledFlags,
+                );
+
+            await projectMigration.migrate();
+            expect(
+              project.xcodeProjectSchemeFile().readAsStringSync(),
+              _validBuildActions(platform, hasFrameworkScript: true, hasBuildEntries: false),
+            );
+          });
+
           testWithoutContext('successfully updates scheme with preexisting PreActions', () async {
             final MemoryFileSystem memoryFileSystem = MemoryFileSystem();
             final BufferLogger testLogger = BufferLogger.test();
@@ -2621,6 +2657,7 @@ String _validBuildActions(
   SupportedPlatform platform, {
   bool hasPreActions = false,
   bool hasFrameworkScript = false,
+  bool hasBuildEntries = true,
 }) {
   final String scriptText;
   if (platform == SupportedPlatform.ios) {
@@ -2656,11 +2693,11 @@ String _validBuildActions(
 \n      <PreActions>
       </PreActions>''';
   }
-  return '''
-   <BuildAction
-      parallelizeBuildables = "YES"
-      buildImplicitDependencies = "YES">$preActions
-      <BuildActionEntries>
+
+  String buildEntries = '';
+  if (hasBuildEntries) {
+    buildEntries = '''
+\n      <BuildActionEntries>
          <BuildActionEntry
             buildForTesting = "YES"
             buildForRunning = "YES"
@@ -2670,7 +2707,23 @@ String _validBuildActions(
 ${_validBuildableReference(platform)}
          </BuildActionEntry>
       </BuildActionEntries>
+''';
+  }
+  return '''
+<Scheme
+   LastUpgradeVersion = "1510"
+   version = "1.3">
+   <BuildAction
+      parallelizeBuildables = "YES"
+      buildImplicitDependencies = "YES">$preActions$buildEntries
    </BuildAction>
+   <LaunchAction>
+      <BuildableProductRunnable
+         runnableDebuggingMode = "0">
+${_validBuildableReference(platform)}
+      </BuildableProductRunnable>
+   </LaunchAction>
+</Scheme>
 ''';
 }