Add call().

lib/src/ast/sass.dart

@@ -19,6 +19,7 @@ export 'sass/expression/number.dart';
 export 'sass/expression/selector.dart';
 export 'sass/expression/string.dart';
 export 'sass/expression/unary_operation.dart';
+export 'sass/expression/value.dart';
 export 'sass/expression/variable.dart';
 export 'sass/interpolation.dart';
 export 'sass/media_query.dart';

lib/src/ast/sass/expression/value.dart

@@ -0,0 +1,26 @@
+// Copyright 2016 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 'package:source_span/source_span.dart';
+
+import '../../../visitor/interface/expression.dart';
+import '../../../value.dart';
+import '../expression.dart';
+
+/// An expression that directly embeds a [Value].
+///
+/// This is never constructed by the parser. It's only used when ASTs are
+/// constructed dynamically, as for the `call()` function.
+class ValueExpression implements Expression {
+  final Value value;
+
+  FileSpan get span => null;
+
+  ValueExpression(this.value);
+
+  /*=T*/ accept/*<T>*/(ExpressionVisitor/*<T>*/ visitor) =>
+      visitor.visitValueExpression(this);
+
+  String toString() => value.toString();
+}

lib/src/functions.dart

@@ -832,6 +832,8 @@ void defineCoreFunctions(Environment environment) {
     var number2 = arguments[1].assertNumber("number2");
     return new SassBoolean(number1.isComparableTo(number2));
   });
+
+  // call() is defined in PerformVisitor to provide it access to private APIs.
 }
 
 num _percentageOrUnitless(SassNumber number, num max, String name) {

lib/src/utils.dart

@@ -61,6 +61,9 @@ int listHash(List list) => const ListEquality().hash(list);
 
 FileSpan spanForList(List<AstNode> nodes) {
   if (nodes.isEmpty) return null;
+  // Spans may be null for dynamically-constructed ASTs.
+  if (nodes.first.span == null) return null;
+  if (nodes.last.span == null) return null;
   return nodes.first.span.expand(nodes.last.span);
 }
 

lib/src/visitor/interface/expression.dart

@@ -16,5 +16,6 @@ abstract class ExpressionVisitor<T> {
   T visitSelectorExpression(SelectorExpression node);
   T visitStringExpression(StringExpression node);
   T visitUnaryOperationExpression(UnaryOperationExpression node);
+  T visitValueExpression(ValueExpression node);
   T visitVariableExpression(VariableExpression node);
 }

lib/src/visitor/perform.dart

@@ -58,7 +58,18 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
 
   PerformVisitor({Iterable<String> loadPaths, Environment environment})
       : _loadPaths = loadPaths == null ? const [] : new List.from(loadPaths),
-        _environment = environment ?? new Environment();
+        _environment = environment ?? new Environment() {
+    _environment.defineFunction("call", r"$name, $args...", (arguments) {
+      var name = arguments[0].assertString("name");
+      var args = arguments[1] as SassArgumentList;
+
+      var expression = new FunctionExpression(
+          new Interpolation([name.text], null),
+          new ArgumentInvocation([], {}, null,
+              rest: new ValueExpression(args)));
+      return expression.accept(this);
+    });
+  }
 
   void visit(node) {
     if (node is Statement) {
@@ -613,6 +624,8 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
     }, node.span);
   }
 
+  Value visitValueExpression(ValueExpression node) => node.value;
+
   Value visitVariableExpression(VariableExpression node) {
     var result = _environment.getVariable(node.name);
     if (result != null) return result;