Add an explicit IfClause class

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

@@ -3,7 +3,6 @@
 // https://opensource.org/licenses/MIT.
 
 import 'package:source_span/source_span.dart';
-import 'package:tuple/tuple.dart';
 
 import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
@@ -18,30 +17,50 @@ class IfRule implements Statement {
   /// The first clause whose expression evaluates to `true` will have its
   /// statements executed. If no expression evaluates to `true`, `lastClause`
   /// will be executed if it's not `null`.
-  final List<Tuple2<Expression, List<Statement>>> clauses;
+  final List<IfClause> clauses;
 
   /// The final, unconditional `@else` clause.
   ///
   /// This is `null` if there is no unconditional `@else`.
-  final List<Statement> lastClause;
+  final IfClause lastClause;
 
   final FileSpan span;
 
-  IfRule(Iterable<Tuple2<Expression, Iterable<Statement>>> clauses, this.span,
-      {Iterable<Statement> lastClause})
-      : clauses = new List.unmodifiable(clauses.map((pair) =>
-            new Tuple2(pair.item1, new List.unmodifiable(pair.item2)))),
-        lastClause =
-            lastClause == null ? null : new List.unmodifiable(lastClause);
+  IfRule(Iterable<IfClause> clauses, this.span, {this.lastClause})
+      : clauses = new List.unmodifiable(clauses) {
+    assert(clauses.every((clause) => clause.expression != null));
+    assert(lastClause?.expression == null);
+  }
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitIfRule(this);
 
   String toString() {
     var first = true;
-    return clauses.map((pair) {
+    return clauses.map((clause) {
       var name = first ? 'if' : 'else';
       first = false;
-      return '@$name ${pair.item1} {${pair.item2.join(" ")}}';
+      return '@$name ${clause.expression} {${clause.children.join(" ")}}';
     }).join(' ');
   }
 }
+
+/// A single clause in an `@if` rule.
+class IfClause {
+  /// The expression to evaluate to determine whether to run this rule, or
+  /// `null` if this is the final unconditional `@else` clause.
+  final Expression expression;
+
+  /// The statements to evaluate if this clause matches.
+  final List<Statement> children;
+
+  IfClause(this.expression, Iterable<Statement> children)
+      : children = new List.unmodifiable(children);
+
+  IfClause.last(Iterable<Statement> children)
+      : expression = null,
+        children = new List.unmodifiable(children);
+
+  String toString() =>
+      (expression == null ? "@else" : "@if $expression") +
+      " {${children.join(' ')}}";
+}

lib/src/parse/stylesheet.dart

@@ -671,16 +671,16 @@ abstract class StylesheetParser extends Parser {
     var expression = _expression();
     var children = this.children(child);
 
-    var clauses = [new Tuple2(expression, children)];
-    List<Statement> lastClause;
+    var clauses = [new IfClause(expression, children)];
+    IfClause lastClause;
 
     while (scanElse(ifIndentation)) {
       whitespace();
       if (scanIdentifier("if")) {
         whitespace();
-        clauses.add(new Tuple2(_expression(), this.children(child)));
+        clauses.add(new IfClause(_expression(), this.children(child)));
       } else {
-        lastClause = this.children(child);
+        lastClause = new IfClause.last(this.children(child));
         break;
       }
     }

lib/src/visitor/async_evaluate.dart

@@ -634,16 +634,17 @@ class _EvaluateVisitor
 
   Future<Value> visitIfRule(IfRule node) async {
     var clause = node.lastClause;
-    for (var pair in node.clauses) {
-      if ((await pair.item1.accept(this)).isTruthy) {
-        clause = pair.item2;
+    for (var clauseToCheck in node.clauses) {
+      if ((await clauseToCheck.expression.accept(this)).isTruthy) {
+        clause = clauseToCheck;
         break;
       }
     }
     if (clause == null) return null;
 
     return await _environment.scope(
-        () => _handleReturn<Statement>(clause, (child) => child.accept(this)),
+        () => _handleReturn<Statement>(
+            clause.children, (child) => child.accept(this)),
         semiGlobal: true);
   }
 

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: ae64ba442752642066f0e9e038f5f2c1bbfa866b
+// Checksum: 33e96540f3e5999ed172d168221b27910e8672b1
 
 import 'dart:math' as math;
 
@@ -629,16 +629,17 @@ class _EvaluateVisitor
 
   Value visitIfRule(IfRule node) {
     var clause = node.lastClause;
-    for (var pair in node.clauses) {
-      if (pair.item1.accept(this).isTruthy) {
-        clause = pair.item2;
+    for (var clauseToCheck in node.clauses) {
+      if (clauseToCheck.expression.accept(this).isTruthy) {
+        clause = clauseToCheck;
         break;
       }
     }
     if (clause == null) return null;
 
     return _environment.scope(
-        () => _handleReturn<Statement>(clause, (child) => child.accept(this)),
+        () => _handleReturn<Statement>(
+            clause.children, (child) => child.accept(this)),
         semiGlobal: true);
   }