Bump kernel and front_end to language version 3.12.

Also remove code that is now marked as dead, thanks to sound flow
analysis.

Change-Id: I08c4f5c3a493d561210e8e557cd458fe6a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/487542
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>

pkg/front_end/pubspec.yaml

@@ -6,7 +6,7 @@ name: front_end
 publish_to: none
 
 environment:
-  sdk: ^3.9.0
+  sdk: '^3.12.0'
 
 workspace:
   - testcases

pkg/kernel/lib/testing/type_parser_environment.dart

@@ -373,15 +373,9 @@ class _KernelFromParsedType implements Visitor<Node, TypeParserEnvironment> {
       TypeParameterType type = new TypeParameterType(
           declaration,
           interpretParsedNullability(node.parsedNullability,
-              ifOmitted: nullability));
-      // If the nullability was omitted on the type and can't be computed from
-      // the bound because it's not yet available, it will be set to null.  In
-      // that case, put it to the list to be updated later, when the bound is
-      // available.
-      // ignore: unnecessary_null_comparison
-      if (type.declaredNullability == null) {
-        environment.pendingNullabilities.add(type);
-      }
+          ifOmitted: nullability,
+        ),
+      );
       return type;
     } else if (declaration is StructuralParameter) {
       if (arguments.isNotEmpty) {
@@ -394,15 +388,9 @@ class _KernelFromParsedType implements Visitor<Node, TypeParserEnvironment> {
       StructuralParameterType type = new StructuralParameterType(
           declaration,
           interpretParsedNullability(node.parsedNullability,
-              ifOmitted: nullability));
-      // If the nullability was omitted on the type and can't be computed from
-      // the bound because it's not yet available, it will be set to null.  In
-      // that case, put it to the list to be updated later, when the bound is
-      // available.
-      // ignore: unnecessary_null_comparison
-      if (type.declaredNullability == null) {
-        environment.pendingNullabilities.add(type);
-      }
+          ifOmitted: nullability,
+        ),
+      );
       return type;
     } else if (declaration is Typedef) {
       return new TypedefType(declaration,

pkg/kernel/pubspec.yaml

@@ -6,7 +6,7 @@ name: kernel
 publish_to: none
 
 environment:
-  sdk: ^3.6.0
+  sdk: '^3.12.0'
 
 resolution: workspace