Revert "--sound-null-safety instead of enable-experiment where possible (#26999)" (#27059)

This reverts commit 3283cafe173808ac0df3ed6c79846eb98c694cf2.
diff --git a/lib/snapshot/BUILD.gn b/lib/snapshot/BUILD.gn
index 0e1173e..f7c2346 100644
--- a/lib/snapshot/BUILD.gn
+++ b/lib/snapshot/BUILD.gn
@@ -46,7 +46,7 @@
   ]
 
   args = [
-    "--sound-null-safety",
+    "--enable-experiment=non-nullable",
     "--snapshot_kind=core",
     "--enable_mirrors=false",
     "--vm_snapshot_data=" + rebase_path(vm_snapshot_data),
diff --git a/testing/run_tests.py b/testing/run_tests.py
index 1ff1139..fa41d30 100755
--- a/testing/run_tests.py
+++ b/testing/run_tests.py
@@ -233,7 +233,8 @@
     dart,
     '--disable-dart-dev',
     frontend_server,
-    '--sound-null-safety',
+    '--enable-experiment=non-nullable',
+    '--no-sound-null-safety',
     '--sdk-root',
     flutter_patched_sdk,
     '--incremental',
diff --git a/testing/testing.gni b/testing/testing.gni
index 579c437..6a2bdc2 100644
--- a/testing/testing.gni
+++ b/testing/testing.gni
@@ -80,7 +80,7 @@
       rebase_path("$root_out_dir/flutter_patched_sdk"),
       "--target",
       "flutter",
-      "--sound-null-safety",
+      "--enable-experiment=non-nullable",
       "--output-dill",
       rebase_path(invoker.dart_kernel, root_out_dir),
       "--depfile",
diff --git a/web_sdk/BUILD.gn b/web_sdk/BUILD.gn
index e6f3e8f..dd83ab9 100644
--- a/web_sdk/BUILD.gn
+++ b/web_sdk/BUILD.gn
@@ -116,7 +116,7 @@
   script = "//third_party/dart/utils/bazel/kernel_worker.dart"
 
   args = [
-    "--sound-null-safety",
+    "--enable-experiment=non-nullable",
     "--summary-only",
     "--target",
     "ddc",
@@ -162,7 +162,7 @@
   ]
 
   args = [
-    "--sound-null-safety",
+    "--enable-experiment=non-nullable",
     "--compile-sdk",
     "dart:core",
 
@@ -215,7 +215,7 @@
   ]
 
   args = [
-    "--sound-null-safety",
+    "--enable-experiment=non-nullable",
     "--compile-sdk",
     "dart:core",
 
@@ -270,7 +270,7 @@
   ]
 
   args = [
-    "--sound-null-safety",
+    "--enable-experiment=non-nullable",
     "--compile-sdk",
     "dart:core",
 
@@ -325,6 +325,7 @@
   ]
 
   args = [
+    "--enable-experiment=non-nullable",
     "--sound-null-safety",
     "--compile-sdk",
     "dart:core",
@@ -379,6 +380,7 @@
   ]
 
   args = [
+    "--enable-experiment=non-nullable",
     "--sound-null-safety",
     "--compile-sdk",
     "dart:core",
@@ -434,6 +436,7 @@
   ]
 
   args = [
+    "--enable-experiment=non-nullable",
     "--sound-null-safety",
     "--compile-sdk",
     "dart:core",
@@ -484,6 +487,7 @@
   script = "//third_party/dart/utils/bazel/kernel_worker.dart"
 
   args = [
+    "--enable-experiment=non-nullable",
     "--sound-null-safety",
     "--summary-only",
     "--target",