Use null-aware operators to clean up the code

pkg/analyzer/lib/dart/ast/ast.dart

Index: pkg/analyzer/lib/dart/ast/ast.dart
diff --git a/pkg/analyzer/lib/dart/ast/ast.dart b/pkg/analyzer/lib/dart/ast/ast.dart
index 6fa9188a337b24e569aadfeb713848b31ea3d416..bd54ec59749723744e8cc8e230f1862664edfe9b 100644
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@@ -4467,8 +4467,8 @@ abstract class ImportDirective extends NamespaceDirective {
     //
     SimpleIdentifier prefix1 = import1.prefix;
     SimpleIdentifier prefix2 = import2.prefix;
-    String prefixStr1 = prefix1 != null ? prefix1.name : null;
-    String prefixStr2 = prefix2 != null ? prefix2.name : null;
+    String prefixStr1 = prefix1?.name;
+    String prefixStr2 = prefix2?.name;
     if (prefixStr1 != null || prefixStr2 != null) {
       if (prefixStr1 == null) {
         return -1;