[ddc] Cleanup non-nullable experiment flag

Also remove dead code that was a workaround when there were
two forks of the SDK libraries but one version the dart:_runtime.

Change-Id: I1192fcc5238fb84ab1cda533bee8bef63d289a2e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/231743
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

pkg/dev_compiler/lib/src/compiler/shared_command.dart

@@ -238,9 +238,6 @@ class SharedCompilerOptions {
     // that build systems do not depend on passing windows-style paths here.
     return p.toUri(moduleName).toString();
   }
-
-  // TODO(nshahan) Cleanup when NNBD graduates experimental status.
-  bool get enableNullSafety => experiments['non-nullable'] ?? false;
 }
 
 /// Finds explicit module names of the form `path=name` in [summaryPaths],

pkg/dev_compiler/lib/src/kernel/command.dart

@@ -276,7 +276,7 @@ Future<CompilerResult> _compile(List<String> args,
         additionalDills,
         DevCompilerTarget(TargetFlags(
             trackWidgetCreation: trackWidgetCreation,
-            enableNullSafety: options.enableNullSafety)),
+            enableNullSafety: options.soundNullSafety)),
         fileSystem: fileSystem,
         explicitExperimentalFlags: explicitExperimentalFlags,
         environmentDefines: declaredVariables,
@@ -315,7 +315,7 @@ Future<CompilerResult> _compile(List<String> args,
         inputDigests,
         DevCompilerTarget(TargetFlags(
             trackWidgetCreation: trackWidgetCreation,
-            enableNullSafety: options.enableNullSafety)),
+            enableNullSafety: options.soundNullSafety)),
         fileSystem: fileSystem,
         explicitExperimentalFlags: explicitExperimentalFlags,
         environmentDefines: declaredVariables,

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -3013,14 +3013,7 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     //   * `class A extends B {...}` where B is the InterfaceType.
     //   * Emitting non-null constructor calls.
     // * The InterfaceType is the Null type.
-    // * The types were written in JS context or as part of the dart:_runtime
-    //   library.
-    if (!emitNullability ||
-        type == _coreTypes.deprecatedNullType ||
-        // TODO(38701) Remove these once the SDK has unforked and is running
-        // "opted-in"
-        !coreLibrary.isNonNullableByDefault &&
-            (_isInForeignJS || isSdkInternalRuntime(currentLibrary))) {
+    if (!emitNullability || type == _coreTypes.deprecatedNullType) {
       return typeRep;
     }