[3.0 alpha][dart2js] No longer infer the null-safety mode from sources.

This prepares dart2js to default to sound null safety in Dart 3.0.

The compiler no longer infers the default mode based on the input
program, instead uses sound null safety unless
'--no-sound-null-safety' is specified.

On a separate change, we expect to remove the option of disabling
null safety.

Note: this change should also be reflected in the CHANGELOG, but
we've currently not included it to redeuce merge conflicts, as we plan
to land multiple 3.0-alpha changes in a short window of time

Change-Id: Id87498cc5976548ec89d1f36c60674b72406950a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/270860
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>

utils/compiler/BUILD.gn

@@ -70,6 +70,9 @@ application_snapshot("dart2js") {
     # Specifying the platform explicitly elides running the CFE on the sdk
     # sources.
     "--platform-binaries=" + rebase_path("$root_out_dir/"),
+
+    # TODO(48820): remove once dart2js is migrated.
+    "--no-sound-null-safety",
     rebase_path("../../pkg/compiler/lib/src/util/memory_compiler.dart"),
   ]
 }