fix devicelab tests (#36717)

dev/devicelab/lib/tasks/perf_tests.dart

@@ -231,7 +231,9 @@ class WebCompileTest {
         '-v',
         '--release',
         '--no-pub',
-      ]);
+      ], environment: <String, String>{
+        'FLUTTER_WEB': 'true',
+      });
       final String output = '${flutterDirectory.path}/examples/hello_world/build/web/main.dart.js';
       await _measureSize('hello_world', output, metrics);
       return null;
@@ -243,7 +245,9 @@ class WebCompileTest {
         '-v',
         '--release',
         '--no-pub',
-      ]);
+      ], environment: <String, String>{
+        'FLUTTER_WEB': 'true',
+      });
       final String output = '${flutterDirectory.path}/examples/flutter_gallery/build/web/main.dart.js';
       await _measureSize('flutter_gallery', output, metrics);
       return null;
@@ -262,7 +266,9 @@ class WebCompileTest {
           '-v',
           '--release',
           '--no-pub',
-        ]);
+        ], environment: <String, String>{
+          'FLUTTER_WEB': 'true',
+        });
         await _measureSize('basic_material_app', path.join(sampleDir.path, 'build/web/main.dart.js'), metrics);
       });
     });