[dart2js] Remove SNS checks flag.

Change-Id: Idb2163a47e7c6ad4f2482bc24aecfc3483c7023b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/416342
Reviewed-by: Nate Biggs <[email protected]>

Author: fishythefish

pkg/compiler/lib/src/commandline_options.dart

@@ -60,9 +60,6 @@ class Flags {
 
   static const String experimentNewRti = '--experiment-new-rti';
 
-  static const String experimentNullSafetyChecks =
-      '--experiment-null-safety-checks';
-
   static const String enableLanguageExperiments = '--enable-experiment';
 
   static const String fastStartup = '--fast-startup';

pkg/compiler/lib/src/dart2js.dart

@@ -552,7 +552,6 @@ Future<api.CompilationResult> compile(
     _OneOption(Flags.experimentToBoolean, passThrough),
     _OneOption(Flags.experimentUnreachableMethodsThrow, passThrough),
     _OneOption(Flags.experimentCallInstrumentation, passThrough),
-    _OneOption(Flags.experimentNullSafetyChecks, ignoreOption),
     _OneOption(Flags.experimentNewRti, ignoreOption),
     _OneOption('${Flags.mergeFragmentsThreshold}=.+', passThrough),