Extend RecursiveStatementVisitor for expressions (#639)

This adds `RecursiveAstVisitor`, which extends
`RecursiveStatementVisitor` to traverse expressions as well.

lib/src/visitor/recursive_ast.dart

@@ -0,0 +1,80 @@
+// Copyright 2019 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import '../ast/sass.dart';
+import 'interface/expression.dart';
+import 'recursive_statement.dart';
+
+/// A visitor that recursively traverses each statement and expression in a Sass
+/// AST.
+///
+/// This extends [RecursiveStatementVisitor] to traverse each expression in
+/// addition to each statement.
+///
+/// The default implementation of the visit methods all return `null`.
+abstract class RecursiveAstVisitor<T> extends RecursiveStatementVisitor<T>
+    implements ExpressionVisitor<T> {
+  void visitExpression(Expression expression) {
+    expression.accept(this);
+  }
+
+  T visitBinaryOperationExpression(BinaryOperationExpression node) {
+    node.left.accept(this);
+    node.right.accept(this);
+    return null;
+  }
+
+  T visitBooleanExpression(BooleanExpression node) => null;
+
+  T visitColorExpression(ColorExpression node) => null;
+
+  T visitFunctionExpression(FunctionExpression node) {
+    visitInterpolation(node.name);
+    visitArgumentInvocation(node.arguments);
+    return null;
+  }
+
+  T visitIfExpression(IfExpression node) {
+    visitArgumentInvocation(node.arguments);
+    return null;
+  }
+
+  T visitListExpression(ListExpression node) {
+    for (var item in node.contents) {
+      item.accept(this);
+    }
+    return null;
+  }
+
+  T visitMapExpression(MapExpression node) {
+    for (var pair in node.pairs) {
+      pair.item1.accept(this);
+      pair.item2.accept(this);
+    }
+    return null;
+  }
+
+  T visitNullExpression(NullExpression node) => null;
+
+  T visitNumberExpression(NumberExpression node) => null;
+
+  T visitParenthesizedExpression(ParenthesizedExpression node) =>
+      node.expression.accept(this);
+
+  T visitSelectorExpression(SelectorExpression node) => null;
+
+  T visitStringExpression(StringExpression node) {
+    visitInterpolation(node.text);
+    return null;
+  }
+
+  T visitUnaryOperationExpression(UnaryOperationExpression node) =>
+      node.operand.accept(this);
+
+  T visitUseRule(UseRule node) => null;
+
+  T visitValueExpression(ValueExpression node) => null;
+
+  T visitVariableExpression(VariableExpression node) => null;
+}

lib/src/visitor/recursive_statement.dart

@@ -137,6 +137,8 @@ abstract class RecursiveStatementVisitor<T> implements StatementVisitor<T> {
     return visitChildren(node);
   }
 
+  T visitUseRule(UseRule node) => null;
+
   T visitVariableDeclaration(VariableDeclaration node) {
     visitExpression(node.expression);
     return null;