print traces when transforming an asset (flutter#146374)

From flutter#143348 (comment):

> before we ship, we should add a printTrace to the tool about each asset transformer we're invoking and the path/arguments we called it with

I think this is a good idea since asset transformers can be arbitrary Dart programs�meaning that a lot can go wrong when running them. For example, they can hang indefinitely or perform some sort of I/O that later results in a tool crash. Knowing that asset transformation was involved when debugging a crash (or a slow/stuck `flutter build`) could be useful, so I think adding a `printTrace` or two is a good idea (or at least not a bad one).

packages/flutter_tools/lib/src/build_system/targets/assets.dart

@@ -150,6 +150,7 @@ Future<Depfile> copyAssets(
                   outputPath: file.path,
                   workingDirectory: environment.projectDir.path,
                   transformerEntries: entry.value.transformers,
+                  logger: environment.logger,
                 );
                 doCopy = false;
                 if (failure != null) {

packages/flutter_tools/lib/src/build_system/tools/asset_transformer.dart

@@ -52,6 +52,7 @@ final class AssetTransformer {
     required String outputPath,
     required String workingDirectory,
     required List<AssetTransformerEntry> transformerEntries,
+    required Logger logger,
   }) async {
 
     String getTempFilePath(int transformStep) {
@@ -65,13 +66,15 @@ final class AssetTransformer {
     File tempOutputFile = _fileSystem.systemTempDirectory.childFile(getTempFilePath(1));
     ErrorHandlingFileSystem.deleteIfExists(tempOutputFile);
 
+    final Stopwatch stopwatch = Stopwatch()..start();
     try {
       for (final (int i, AssetTransformerEntry transformer) in transformerEntries.indexed) {
         final AssetTransformationFailure? transformerFailure = await _applyTransformer(
           asset: tempInputFile,
           output: tempOutputFile,
           transformer: transformer,
           workingDirectory: workingDirectory,
+          logger: logger,
         );
 
         if (transformerFailure != null) {
@@ -91,6 +94,8 @@ final class AssetTransformer {
           ErrorHandlingFileSystem.deleteIfExists(tempOutputFile);
         }
       }
+
+      logger.printTrace("Finished transforming asset at path '${asset.path}' (${stopwatch.elapsedMilliseconds}ms)");
     } finally {
       ErrorHandlingFileSystem.deleteIfExists(tempInputFile);
       ErrorHandlingFileSystem.deleteIfExists(tempOutputFile);
@@ -104,6 +109,7 @@ final class AssetTransformer {
     required File output,
     required AssetTransformerEntry transformer,
     required String workingDirectory,
+    required Logger logger,
   }) async {
     final List<String> transformerArguments = <String>[
       '--input=${asset.absolute.path}',
@@ -118,6 +124,7 @@ final class AssetTransformer {
       ...transformerArguments,
     ];
 
+    logger.printTrace("Transforming asset using command '${command.join(' ')}'");
     final ProcessResult result = await _processManager.run(
       command,
       workingDirectory: workingDirectory,
@@ -199,6 +206,7 @@ final class DevelopmentAssetTransformer {
         outputPath: output.path,
         transformerEntries: transformerEntries,
         workingDirectory: workingDirectory,
+        logger: _logger,
       );
       if (failure != null) {
         _logger.printError(failure.message);

packages/flutter_tools/lib/src/bundle_builder.dart

@@ -209,6 +209,7 @@ Future<void> writeBundle(
               outputPath: file.path,
               workingDirectory: projectDir.path,
               transformerEntries: entry.value.transformers,
+              logger: logger,
             );
             doCopy = false;
             if (failure != null) {

packages/flutter_tools/test/general.shard/build_system/targets/asset_transformer_test.dart

@@ -71,6 +71,7 @@ void main() {
           ],
         )
       ],
+      logger: logger,
     );
 
     expect(transformationFailure, isNull, reason: logger.errorText);
@@ -127,6 +128,7 @@ void main() {
           args: <String>[],
         )
       ],
+      logger: BufferLogger.test(),
     );
 
     expect(asset, exists);
@@ -187,6 +189,7 @@ Something went wrong''');
           args: <String>[],
         )
       ],
+      logger: BufferLogger.test(),
     );
 
     expect(processManager, hasNoRemainingExpectations);
@@ -286,6 +289,7 @@ Transformation failed, but I forgot to exit with a non-zero code.'''
           args: <String>[],
         ),
       ],
+      logger: BufferLogger.test(),
     );
 
     expect(processManager, hasNoRemainingExpectations);
@@ -364,6 +368,7 @@ Transformation failed, but I forgot to exit with a non-zero code.'''
           args: <String>[],
         ),
       ],
+      logger: BufferLogger.test(),
     );
 
     expect(failure, isNotNull);