Remove some dead code (#1565)

This change preserves behavior, though the current behavior has the
appearance of a bug so an author more familiar with the intention may
want to add a test.

There are no assignments to `first` so the conditional can be removed.
When building internally the analyze is surfacing a `dead_code`
diagnostic which needs to be ignored. Fixing the code will allow us to
remove the ignore.

I don't know why the diagnostic does not show up when analyzing using
the external analyzer.
dart-lang/sdk#47837

lib/src/ast/sass/statement/if_rule.dart

@@ -2,6 +2,7 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'package:collection/collection.dart';
 import 'package:meta/meta.dart';
 import 'package:source_span/source_span.dart';
 
@@ -41,10 +42,9 @@ class IfRule implements Statement {
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitIfRule(this);
 
   String toString() {
-    var first = true;
     var result = clauses
-        .map((clause) =>
-            "@${first ? 'if' : 'else if'} {${clause.children.join(' ')}}")
+        .mapIndexed((index, clause) =>
+            "@${index == 0 ? 'if' : 'else if'} {${clause.children.join(' ')}}")
         .join(' ');
 
     var lastClause = this.lastClause;