Add a separate Environment method to create dummy import modules

This avoids triggering `assert(!atRoot)` for nested environments,
while still having that precaution for normal `toModule()` calls.

lib/src/async_environment.dart

@@ -810,6 +810,20 @@ class AsyncEnvironment {
         forwarded: _forwardedModules);
   }
 
+  /// Returns a module with the same members and upstream modules as [this], but
+  /// an empty stylesheet and extender.
+  ///
+  /// This is used when resolving imports, since they need to inject forwarded
+  /// members into the current scope. It's the only situation in which a nested
+  /// environment can become a module.
+  Module toDummyModule() {
+    return _EnvironmentModule(
+        this,
+        CssStylesheet(const [], SourceFile.decoded(const []).span(0)),
+        Extender.empty,
+        forwarded: _forwardedModules);
+  }
+
   /// Returns the module with the given [namespace], or throws a
   /// [SassScriptException] if none exists.
   Module _getModule(String namespace) {

lib/src/environment.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_environment.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 76e7c2e929fc7d19390987069ff3a8c4b3fadfc5
+// Checksum: b5212ffc7c50a8e7e436b25c7c16eb2996da2def
 //
 // ignore_for_file: unused_import
 
@@ -815,6 +815,20 @@ class Environment {
         forwarded: _forwardedModules);
   }
 
+  /// Returns a module with the same members and upstream modules as [this], but
+  /// an empty stylesheet and extender.
+  ///
+  /// This is used when resolving imports, since they need to inject forwarded
+  /// members into the current scope. It's the only situation in which a nested
+  /// environment can become a module.
+  Module<Callable> toDummyModule() {
+    return _EnvironmentModule(
+        this,
+        CssStylesheet(const [], SourceFile.decoded(const []).span(0)),
+        Extender.empty,
+        forwarded: _forwardedModules);
+  }
+
   /// Returns the module with the given [namespace], or throws a
   /// [SassScriptException] if none exists.
   Module<Callable> _getModule(String namespace) {

lib/src/visitor/async_evaluate.dart

@@ -1420,8 +1420,7 @@ class _EvaluateVisitor
       // Create a dummy module with empty CSS and no extensions to make forwarded
       // members available in the current import context and to combine all the
       // CSS from modules used by [stylesheet].
-      var module = environment.toModule(
-          CssStylesheet(const [], stylesheet.span), Extender.empty);
+      var module = environment.toDummyModule();
       _environment.importForwards(module);
 
       if (module.transitivelyContainsCss) {

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: f6a7ece88049e365d88091c14cc8143335a06d75
+// Checksum: 0ad7cedc5ee1d05297c4a8d321d492854742e8ab
 //
 // ignore_for_file: unused_import
 
@@ -1417,8 +1417,7 @@ class _EvaluateVisitor
       // Create a dummy module with empty CSS and no extensions to make forwarded
       // members available in the current import context and to combine all the
       // CSS from modules used by [stylesheet].
-      var module = environment.toModule(
-          CssStylesheet(const [], stylesheet.span), Extender.empty);
+      var module = environment.toDummyModule();
       _environment.importForwards(module);
 
       if (module.transitivelyContainsCss) {