Adds impeller backend to skia gold client (flutter#181503)

flutter@1418863
made dart tests run against metal as well as vulkan. At the time there
was no reported error. I suspect the vulkan and metal results are
clobbering each other though since the backend was not part of the
dimensions.

test exempt: is test

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Author: gaaclarke

engine/src/flutter/testing/dart/goldens.dart

@@ -33,7 +33,10 @@ class ImageComparer {
     final workDirectory = Directory(
       impellerEnabled ? '${workDirectoryPath}_iplr' : workDirectoryPath,
     )..createSync();
-    final dimensions = <String, String>{'impeller_enabled': impellerEnabled.toString()};
+    final dimensions = <String, String>{
+      'impeller_enabled': impellerEnabled.toString(),
+      'impeller_backend': impellerBackend ?? 'none',
+    };
     final SkiaGoldClient client = SkiaGoldClient.isAvailable() && _useSkiaGold
         ? SkiaGoldClient(workDirectory, dimensions: dimensions, verbose: verbose)
         : _FakeSkiaGoldClient(workDirectory, dimensions, verbose: verbose);

engine/src/flutter/testing/dart/impeller_enabled.dart

@@ -5,3 +5,19 @@
 import 'dart:io';
 
 bool get impellerEnabled => Platform.executableArguments.contains('--enable-impeller');
+
+String? get impellerBackend {
+  if (!impellerEnabled) {
+    return null;
+  }
+  const backendFlag = '--impeller-backend=';
+  for (final String arg in Platform.executableArguments) {
+    if (arg.startsWith(backendFlag)) {
+      return arg.substring(backendFlag.length);
+    }
+  }
+  if (Platform.isMacOS || Platform.isIOS) {
+    return 'metal';
+  }
+  return 'vulkan';
+}

engine/src/flutter/testing/run_tests.py

@@ -929,6 +929,9 @@ def gather_dart_tests(
   dart_vm_service_tests = glob.glob(f'{dart_tests_dir}/vm_service/*_test.dart')
   dart_tests = glob.glob(f'{dart_tests_dir}/*_test.dart')
 
+  impeller_backends = ['', 'vulkan']
+  if is_mac():
+    impeller_backends.append('metal')
   if 'release' not in build_dir:
     for dart_test_file in dart_vm_service_tests:
       dart_test_basename = os.path.basename(dart_test_file)
@@ -937,7 +940,7 @@ def gather_dart_tests(
       else:
         _logger.info("Gathering dart test '%s' with VM service enabled", dart_test_file)
         for multithreaded in [False, True]:
-          for impeller in ['', 'vulkan', 'metal']:
+          for impeller in impeller_backends:
             yield gather_dart_test(
                 build_dir, dart_test_file,
                 FlutterTesterOptions(
@@ -955,7 +958,7 @@ def gather_dart_tests(
     else:
       _logger.info("Gathering dart test '%s'", dart_test_file)
       for multithreaded in [False, True]:
-        for impeller in ['', 'vulkan', 'metal']:
+        for impeller in impeller_backends:
           yield gather_dart_test(
               build_dir, dart_test_file,
               FlutterTesterOptions(multithreaded=multithreaded, impeller_backend=impeller)