revert embedded protocol changes

lib/src/embedded/dispatcher.dart

@@ -18,7 +18,6 @@ import 'host_callable.dart';
 import 'importer/file.dart';
 import 'importer/host.dart';
 import 'logger.dart';
-import 'mixin_registry.dart';
 import 'util/proto_extensions.dart';
 import 'utils.dart';
 
@@ -111,7 +110,6 @@ final class Dispatcher {
   OutboundMessage_CompileResponse _compile(
       InboundMessage_CompileRequest request) {
     var functions = FunctionRegistry();
-    var mixins = MixinRegistry();
 
     var style = request.style == OutputStyle.COMPRESSED
         ? sass.OutputStyle.compressed
@@ -125,7 +123,7 @@ final class Dispatcher {
           (throw mandatoryError("Importer.importer")));
 
       var globalFunctions = request.globalFunctions
-          .map((signature) => hostCallable(this, functions, mixins, signature));
+          .map((signature) => hostCallable(this, functions, signature));
 
       late sass.CompileResult result;
       switch (request.whichInput()) {

lib/src/embedded/host_callable.dart

@@ -2,7 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import '../../src/embedded/mixin_registry.dart';
 import '../callable.dart';
 import '../exception.dart';
 import 'dispatcher.dart';
@@ -19,12 +18,12 @@ import 'utils.dart';
 /// the name defined in the [signature].
 ///
 /// Throws a [SassException] if [signature] is invalid.
-Callable hostCallable(Dispatcher dispatcher, FunctionRegistry functions,
-    MixinRegistry mixins, String signature,
+Callable hostCallable(
+    Dispatcher dispatcher, FunctionRegistry functions, String signature,
     {int? id}) {
   late Callable callable;
   callable = Callable.fromSignature(signature, (arguments) {
-    var protofier = Protofier(dispatcher, functions, mixins);
+    var protofier = Protofier(dispatcher, functions);
     var request = OutboundMessage_FunctionCallRequest()
       ..arguments.addAll(
           [for (var argument in arguments) protofier.protofy(argument)]);

lib/src/embedded/protofier.dart

@@ -9,7 +9,6 @@ import 'dispatcher.dart';
 import 'embedded_sass.pb.dart' as proto;
 import 'embedded_sass.pb.dart' hide Value, ListSeparator, CalculationOperator;
 import 'function_registry.dart';
-import 'mixin_registry.dart';
 import 'host_callable.dart';
 import 'utils.dart';
 
@@ -24,9 +23,6 @@ final class Protofier {
   /// The IDs of first-class functions.
   final FunctionRegistry _functions;
 
-  /// The IDs of first-class mixins.
-  final MixinRegistry _mixins;
-
   /// Any argument lists transitively contained in [value].
   ///
   /// The IDs of the [Value_ArgumentList] protobufs are always one greater than
@@ -39,10 +35,7 @@ final class Protofier {
   ///
   /// The [functions] tracks the IDs of first-class functions so that the host
   /// can pass them back to the compiler.
-  ///
-  /// Similarly, the [mixins] tracks the IDs of first-class mixins so that the
-  /// host can pass them back to the compiler.
-  Protofier(this._dispatcher, this._functions, this._mixins);
+  Protofier(this._dispatcher, this._functions);
 
   /// Converts [value] to its protocol buffer representation.
   proto.Value protofy(Value value) {
@@ -92,8 +85,6 @@ final class Protofier {
         result.calculation = _protofyCalculation(value);
       case SassFunction():
         result.compilerFunction = _functions.protofy(value);
-      case SassMixin():
-        result.compilerMixin = _mixins.protofy(value);
       case sassTrue:
         result.singleton = SingletonValue.TRUE;
       case sassFalse:
@@ -249,15 +240,9 @@ final class Protofier {
 
         case Value_Value.hostFunction:
           return SassFunction(hostCallable(
-              _dispatcher, _functions, _mixins, value.hostFunction.signature,
+              _dispatcher, _functions, value.hostFunction.signature,
               id: value.hostFunction.id));
 
-        case Value_Value.compilerMixin:
-          var id = value.compilerMixin.id;
-          if (_mixins[id] case var mixin?) return mixin;
-          throw paramsError(
-              "CompilerMixin.id $id doesn't match any known mixins");
-
         case Value_Value.calculation:
           return _deprotofyCalculation(value.calculation);