Add support for @forward

See sass/sass#1094
See #554

lib/src/ast/sass.dart

@@ -40,6 +40,7 @@ export 'sass/statement/each_rule.dart';
 export 'sass/statement/error_rule.dart';
 export 'sass/statement/extend_rule.dart';
 export 'sass/statement/for_rule.dart';
+export 'sass/statement/forward_rule.dart';
 export 'sass/statement/function_rule.dart';
 export 'sass/statement/if_rule.dart';
 export 'sass/statement/import_rule.dart';

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

@@ -0,0 +1,126 @@
+// 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 'package:collection/collection.dart';
+import 'package:source_span/source_span.dart';
+
+import '../../../utils.dart';
+import '../../../visitor/interface/statement.dart';
+import '../expression/string.dart';
+import '../statement.dart';
+
+/// A `@forward` rule.
+class ForwardRule implements Statement {
+  /// The URI of the module to forward.
+  ///
+  /// If this is relative, it's relative to the containing file.
+  final Uri url;
+
+  /// The set of mixin and function names that may be accessed from the
+  /// forwarded module.
+  ///
+  /// If this is empty, no mixins or functions may be accessed. If it's `null`,
+  /// it imposes no restrictions on which mixins and function may be accessed.
+  ///
+  /// If this is non-`null`, [hiddenMixinsAndFunctions] and [hiddenVariables]
+  /// are guaranteed to both be `null` and [shownVariables] is guaranteed to be
+  /// non-`null`.
+  final Set<String> shownMixinsAndFunctions;
+
+  /// The set of variable names (without `$`) that may be accessed from the
+  /// forwarded module.
+  ///
+  /// If this is empty, no variables may be accessed. If it's `null`, it imposes
+  /// no restrictions on which variables may be accessed.
+  ///
+  /// If this is non-`null`, [hiddenMixinsAndFunctions] and [hiddenVariables]
+  /// are guaranteed to both be `null` and [shownMixinsAndFunctions] is
+  /// guaranteed to be non-`null`.
+  final Set<String> shownVariables;
+
+  /// The set of mixin and function names that may not be accessed from the
+  /// forwarded module.
+  ///
+  /// If this is empty, any mixins or functions may be accessed. If it's `null`,
+  /// it imposes no restrictions on which mixins or functions may be accessed.
+  ///
+  /// If this is non-`null`, [shownMixinsAndFunctions] and [shownVariables] are
+  /// guaranteed to both be `null` and [hiddenVariables] is guaranteed to be
+  /// non-`null`.
+  final Set<String> hiddenMixinsAndFunctions;
+
+  /// The set of variable names (without `$`) that may be accessed from the
+  /// forwarded module.
+  ///
+  /// If this is empty, any variables may be accessed. If it's `null`, it
+  /// imposes no restrictions on which variables may be accessed.
+  ///
+  /// If this is non-`null`, [shownMixinsAndFunctions] and [shownVariables] are
+  /// guaranteed to both be `null` and [hiddenMixinsAndFunctions] is guaranteed
+  /// to be non-`null`.
+  final Set<String> hiddenVariables;
+
+  /// The prefix to add to the beginning of the names of members of the used
+  /// module, or `null` if member names are used as-is.
+  final String prefix;
+
+  final FileSpan span;
+
+  /// Creates a `@forward` rule that allows all members to be accessed.
+  ForwardRule(this.url, this.span, {this.prefix})
+      : shownMixinsAndFunctions = null,
+        shownVariables = null,
+        hiddenMixinsAndFunctions = null,
+        hiddenVariables = null;
+
+  /// Creates a `@forward` rule that allows only members included in
+  /// [shownMixinsAndFunctions] and [shownVariables] to be accessed.
+  ForwardRule.show(this.url, Iterable<String> shownMixinsAndFunctions,
+      Iterable<String> shownVariables, this.span,
+      {this.prefix})
+      : shownMixinsAndFunctions =
+            UnmodifiableSetView(normalizedSet(shownMixinsAndFunctions)),
+        shownVariables = UnmodifiableSetView(normalizedSet(shownVariables)),
+        hiddenMixinsAndFunctions = null,
+        hiddenVariables = null;
+
+  /// Creates a `@forward` rule that allows only members not included in
+  /// [hiddenMixinsAndFunctions] and [hiddenVariables] to be accessed.
+  ForwardRule.hide(this.url, Iterable<String> hiddenMixinsAndFunctions,
+      Iterable<String> hiddenVariables, this.span,
+      {this.prefix})
+      : shownMixinsAndFunctions = null,
+        shownVariables = null,
+        hiddenMixinsAndFunctions =
+            UnmodifiableSetView(normalizedSet(hiddenMixinsAndFunctions)),
+        hiddenVariables = UnmodifiableSetView(normalizedSet(hiddenVariables));
+
+  T accept<T>(StatementVisitor<T> visitor) => visitor.visitForwardRule(this);
+
+  String toString() {
+    var buffer =
+        StringBuffer("@forward ${StringExpression.quoteText(url.toString())}");
+
+    if (shownMixinsAndFunctions != null) {
+      buffer
+        ..write(" show ")
+        ..write(_memberList(shownMixinsAndFunctions, shownVariables));
+    } else if (hiddenMixinsAndFunctions != null) {
+      buffer
+        ..write(" hide ")
+        ..write(_memberList(hiddenMixinsAndFunctions, hiddenVariables));
+    }
+
+    if (prefix != null) buffer.write(" as $prefix*");
+    buffer.write(";");
+    return buffer.toString();
+  }
+
+  /// Returns a combined list of names of the given members.
+  String _memberList(
+          Iterable<String> mixinsAndFunctions, Iterable<String> variables) =>
+      shownMixinsAndFunctions
+          .followedBy(shownVariables.map((name) => "\$$name"))
+          .join(", ");
+}