Fix greatest/least closure computation when there are multiple `?`s.

pkg/front_end/lib/src/fasta/type_inference/type_schema_elimination.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_schema_elimination.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_schema_elimination.dart b/pkg/front_end/lib/src/fasta/type_inference/type_schema_elimination.dart
index 0c96c24d49bf0691e3f661b68227972782dd4303..a5bb0eb183b6979f9ffc44b2121920f28926c215 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_schema_elimination.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_schema_elimination.dart
@@ -59,7 +59,7 @@ class _TypeSchemaEliminationVisitor extends TypeSchemaVisitor<DartType> {
     for (int i = 0; i < node.positionalParameters.length; i++) {
       DartType substitution = node.positionalParameters[i].accept(this);
       if (substitution != null) {
-        newPositionalParameters =
+        newPositionalParameters ??=
             node.positionalParameters.toList(growable: false);
         newPositionalParameters[i] = substitution;
       }
@@ -68,7 +68,7 @@ class _TypeSchemaEliminationVisitor extends TypeSchemaVisitor<DartType> {
     for (int i = 0; i < node.namedParameters.length; i++) {
       DartType substitution = node.namedParameters[i].type.accept(this);
       if (substitution != null) {
-        newNamedParameters = node.namedParameters.toList(growable: false);
+        newNamedParameters ??= node.namedParameters.toList(growable: false);
         newNamedParameters[i] =
             new NamedType(node.namedParameters[i].name, substitution);
       }