Delete ModifiableCssNode.modifyChildren

Instead of doing this, we now construct a new (unmodifiable)
CssStylesheet if additional imports need to be added.

lib/src/ast/css/modifiable/node.dart

@@ -93,14 +93,6 @@ abstract class ModifiableCssParentNode extends ModifiableCssNode
   /// Returns a copy of [this] with an empty [children] list.
   ModifiableCssParentNode copyWithoutChildren();
 
-  /// Passes a modifiable view of [children] to [modify].
-  ///
-  /// This is used to explicitly indicate when modifications are intended so
-  /// that [children] can remain unmodifiable by default.
-  void modifyChildren(void modify(List<ModifiableCssNode> children)) {
-    modify(_children);
-  }
-
   /// Adds [child] as a child of this statement.
   void addChild(ModifiableCssNode child) {
     child._parent = this;

lib/src/ast/css/stylesheet.dart

@@ -2,9 +2,23 @@
 // 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/css.dart';
 import 'node.dart';
 
 /// A plain CSS stylesheet.
 ///
 /// This is the root plain CSS node. It contains top-level statements.
-abstract class CssStylesheet extends CssParentNode {}
+class CssStylesheet extends CssParentNode {
+  final List<CssNode> children;
+  final FileSpan span;
+  bool get isGroupEnd => false;
+  bool get isChildless => false;
+
+  /// Creates an unmodifiable stylesheet containing [children].
+  CssStylesheet(Iterable<CssNode> children, this.span)
+      : children = List.unmodifiable(children);
+
+  T accept<T>(CssVisitor<T> visitor) => visitor.visitStylesheet(this);
+}

lib/src/visitor/async_evaluate.dart

@@ -327,7 +327,20 @@ class _EvaluateVisitor
 
     await visitStylesheet(node);
 
-    return EvaluateResult(_root, _includedFiles);
+    CssStylesheet stylesheet = _root;
+    if (_outOfOrderImports.isNotEmpty) {
+      // Create a copy of [_root.children] with [_outOfOrderImports] inserted at
+      // [_endOfImports].
+      var statements =
+          List<CssNode>(_root.children.length + _outOfOrderImports.length);
+      statements.setRange(0, _endOfImports, _root.children);
+      statements.setAll(_endOfImports, _outOfOrderImports);
+      statements.setRange(_endOfImports + _outOfOrderImports.length,
+          statements.length, _root.children, _endOfImports);
+      stylesheet = CssStylesheet(statements, _root.span);
+    }
+
+    return EvaluateResult(stylesheet, _includedFiles);
   }
 
   // ## Statements
@@ -340,12 +353,6 @@ class _EvaluateVisitor
       await child.accept(this);
     }
 
-    if (_outOfOrderImports.isNotEmpty) {
-      _root.modifyChildren((children) {
-        children.insertAll(_endOfImports, _outOfOrderImports);
-      });
-    }
-
     _extender.finalize();
     return null;
   }

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: f90160d7f7fd8d5b9aacd1019cfe9e41385d34f5
+// Checksum: 816959e86ecf3e10aaa0ced8a58a35cf3604b3b2
 //
 // ignore_for_file: unused_import
 
@@ -334,7 +334,20 @@ class _EvaluateVisitor
 
     visitStylesheet(node);
 
-    return EvaluateResult(_root, _includedFiles);
+    CssStylesheet stylesheet = _root;
+    if (_outOfOrderImports.isNotEmpty) {
+      // Create a copy of [_root.children] with [_outOfOrderImports] inserted at
+      // [_endOfImports].
+      var statements =
+          List<CssNode>(_root.children.length + _outOfOrderImports.length);
+      statements.setRange(0, _endOfImports, _root.children);
+      statements.setAll(_endOfImports, _outOfOrderImports);
+      statements.setRange(_endOfImports + _outOfOrderImports.length,
+          statements.length, _root.children, _endOfImports);
+      stylesheet = CssStylesheet(statements, _root.span);
+    }
+
+    return EvaluateResult(stylesheet, _includedFiles);
   }
 
   // ## Statements
@@ -347,12 +360,6 @@ class _EvaluateVisitor
       child.accept(this);
     }
 
-    if (_outOfOrderImports.isNotEmpty) {
-      _root.modifyChildren((children) {
-        children.insertAll(_endOfImports, _outOfOrderImports);
-      });
-    }
-
     _extender.finalize();
     return null;
   }