Avoid hard coding "build" for test output files (#79322)

dev/devicelab/lib/tasks/perf_tests.dart

@@ -16,6 +16,11 @@ import 'package:flutter_devicelab/framework/host_agent.dart';
 import 'package:flutter_devicelab/framework/task_result.dart';
 import 'package:flutter_devicelab/framework/utils.dart';
 
+/// Must match flutter_driver/lib/src/common.dart.
+///
+/// Redefined here to avoid taking a dependency on flutter_driver.
+final String _testOutputDirectory = Platform.environment['FLUTTER_TEST_OUTPUTS_DIR'] ?? 'build';
+
 TaskFunction createComplexLayoutScrollPerfTest({bool measureCpuGpu = true}) {
   return PerfTest(
     '${flutterDirectory.path}/dev/benchmarks/complex_layout',
@@ -297,7 +302,7 @@ TaskFunction createStackSizeTest() {
         hostAgent.dumpDirectory.path,
       ]);
       final Map<String, dynamic> data = json.decode(
-        file('$testDirectory/build/stack_size.json').readAsStringSync(),
+        file('$testDirectory/$_testOutputDirectory/stack_size.json').readAsStringSync(),
       ) as Map<String, dynamic>;
 
       final Map<String, dynamic> result = <String, dynamic>{
@@ -394,7 +399,7 @@ TaskFunction createsScrollSmoothnessPerfTest() {
         hostAgent.dumpDirectory.path,
       ]);
       final Map<String, dynamic> data = json.decode(
-        file('$testDirectory/build/scroll_smoothness_test.json').readAsStringSync(),
+        file('$testDirectory/$_testOutputDirectory/scroll_smoothness_test.json').readAsStringSync(),
       ) as Map<String, dynamic>;
 
       final Map<String, dynamic> result = <String, dynamic>{};
@@ -445,7 +450,7 @@ TaskFunction createFramePolicyIntegrationTest() {
         hostAgent.dumpDirectory.path,
       ]);
       final Map<String, dynamic> data = json.decode(
-        file('$testDirectory/build/frame_policy_event_delay.json').readAsStringSync(),
+        file('$testDirectory/$_testOutputDirectory/frame_policy_event_delay.json').readAsStringSync(),
       ) as Map<String, dynamic>;
       final Map<String, dynamic> fullLiveData = data['fullyLive'] as Map<String, dynamic>;
       final Map<String, dynamic> benchmarkLiveData = data['benchmarkLive'] as Map<String, dynamic>;
@@ -557,7 +562,7 @@ class StartupTest {
          ], canFail: true);
         if (result == 0) {
           final Map<String, dynamic> data = json.decode(
-            file('$testDirectory/build/start_up_info.json').readAsStringSync(),
+            file('$testDirectory/$_testOutputDirectory/start_up_info.json').readAsStringSync(),
           ) as Map<String, dynamic>;
           results.add(data);
         } else {
@@ -700,7 +705,7 @@ class PerfTest {
         hostAgent.dumpDirectory.path,
       ]);
       final Map<String, dynamic> data = json.decode(
-        file('$testDirectory/build/$resultFilename.json').readAsStringSync(),
+        file('$testDirectory/$_testOutputDirectory/$resultFilename.json').readAsStringSync(),
       ) as Map<String, dynamic>;
 
       if (data['frame_count'] as int < 5) {
@@ -717,7 +722,7 @@ class PerfTest {
         data,
         detailFiles: <String>[
           if (saveTraceFile)
-            '$testDirectory/build/$traceFilename.json',
+            '$testDirectory/$_testOutputDirectory/$traceFilename.json',
         ],
         benchmarkScoreKeys: benchmarkScoreKeys ?? <String>[
           ..._kCommonScoreKeys,