diff --git a/pkg/front_end/lib/src/source/builder_factory.dart b/pkg/front_end/lib/src/source/builder_factory.dart
index f87cb2e..21c7c0b 100644
--- a/pkg/front_end/lib/src/source/builder_factory.dart
+++ b/pkg/front_end/lib/src/source/builder_factory.dart
@@ -66,11 +66,7 @@
   void registerUnresolvedStructuralVariables(
       List<StructuralVariableBuilder> unboundTypeVariables);
 
-  Iterable<Builder> get members;
-
-  Iterable<Builder> get setters;
-
-  Iterable<ExtensionBuilder> get extensions;
+  Iterable<Builder> get builders;
 
   List<LibraryPart> get libraryParts;
 }
diff --git a/pkg/front_end/lib/src/source/source_builder_factory.dart b/pkg/front_end/lib/src/source/source_builder_factory.dart
index fe615aa..3deaa36 100644
--- a/pkg/front_end/lib/src/source/source_builder_factory.dart
+++ b/pkg/front_end/lib/src/source/source_builder_factory.dart
@@ -34,19 +34,15 @@
         lateMask,
         namedMixinApplicationMask,
         staticMask;
-import '../base/name_space.dart';
-import '../base/problems.dart' show internalProblem, unexpected, unhandled;
+import '../base/problems.dart' show internalProblem, unhandled;
 import '../base/scope.dart';
-import '../base/uri_offset.dart';
 import '../base/uris.dart';
 import '../builder/builder.dart';
 import '../builder/constructor_reference_builder.dart';
 import '../builder/declaration_builders.dart';
 import '../builder/formal_parameter_builder.dart';
-import '../builder/function_builder.dart';
 import '../builder/function_type_builder.dart';
 import '../builder/library_builder.dart';
-import '../builder/member_builder.dart';
 import '../builder/metadata_builder.dart';
 import '../builder/mixin_application_builder.dart';
 import '../builder/named_type_builder.dart';
@@ -90,7 +86,7 @@
   // the outline builder.
   final SourceLibraryBuilder _parent;
 
-  final TypeParameterScopeBuilder _libraryTypeParameterScopeBuilder;
+  final LibraryNameSpaceBuilder _libraryNameSpaceBuilder;
 
   /// Index of the library we use references for.
   final IndexedLibrary? indexedLibrary;
@@ -114,9 +110,6 @@
   @override
   final List<Export> exports = <Export>[];
 
-  /// List of [PrefixBuilder]s for imports with prefixes.
-  List<PrefixBuilder>? _prefixBuilders;
-
   /// Map from synthesized names used for omitted types to their corresponding
   /// synthesized type declarations.
   ///
@@ -136,8 +129,6 @@
 
   final LookupScope _scope;
 
-  final NameSpace _nameSpace;
-
   /// Index for building unique lowered names for wildcard variables.
   int wildcardVariableIndex = 0;
 
@@ -156,20 +147,18 @@
       {required SourceCompilationUnit compilationUnit,
       required SourceLibraryBuilder augmentationRoot,
       required SourceLibraryBuilder parent,
-      required TypeParameterScopeBuilder libraryTypeParameterScopeBuilder,
+      required LibraryNameSpaceBuilder libraryNameSpaceBuilder,
       required ProblemReporting problemReporting,
       required LookupScope scope,
-      required NameSpace nameSpace,
       required LibraryName libraryName,
       required IndexedLibrary? indexedLibrary,
       required Map<String, Builder>? omittedTypeDeclarationBuilders})
       : _compilationUnit = compilationUnit,
         _augmentationRoot = augmentationRoot,
-        _libraryTypeParameterScopeBuilder = libraryTypeParameterScopeBuilder,
+        _libraryNameSpaceBuilder = libraryNameSpaceBuilder,
         _problemReporting = problemReporting,
         _parent = parent,
         _scope = scope,
-        _nameSpace = nameSpace,
         libraryName = libraryName,
         indexedLibrary = indexedLibrary,
         _omittedTypeDeclarationBuilders = omittedTypeDeclarationBuilders,
@@ -2825,110 +2814,8 @@
   Builder _addBuilderToLibrary(
       String name, Builder declaration, int charOffset) {
     assert(_declarationFragments.isEmpty);
-    if (declaration is SourceExtensionBuilder &&
-        declaration.isUnnamedExtension) {
-      declaration.parent = _parent;
-      _libraryTypeParameterScopeBuilder.extensions.add(declaration);
-      return declaration;
-    }
-
-    if (declaration is MemberBuilder) {
-      declaration.parent = _parent;
-    } else if (declaration is TypeDeclarationBuilder) {
-      declaration.parent = _parent;
-    } else if (declaration is PrefixBuilder) {
-      assert(declaration.parent == _parent);
-    } else {
-      return unhandled("${declaration.runtimeType}", "addBuilder", charOffset,
-          _compilationUnit.fileUri);
-    }
-
-    assert(
-        !(declaration is FunctionBuilder &&
-            (declaration.isConstructor || declaration.isFactory)),
-        // Coverage-ignore(suite): Not run.
-        "Unexpected constructor in library: $declaration.");
-
-    Map<String, Builder> members = declaration.isSetter
-        ? _libraryTypeParameterScopeBuilder.setters
-        : _libraryTypeParameterScopeBuilder.members;
-
-    Builder? existing = members[name];
-
-    if (existing == declaration) return declaration;
-
-    if (declaration.next != null && declaration.next != existing) {
-      unexpected(
-          "${declaration.next!.fileUri}@${declaration.next!.charOffset}",
-          "${existing?.fileUri}@${existing?.charOffset}",
-          declaration.charOffset,
-          declaration.fileUri);
-    }
-    declaration.next = existing;
-    if (declaration is PrefixBuilder && existing is PrefixBuilder) {
-      assert(existing.next is! PrefixBuilder);
-      Builder? deferred;
-      Builder? other;
-      if (declaration.deferred) {
-        deferred = declaration;
-        other = existing;
-      } else if (existing.deferred) {
-        deferred = existing;
-        other = declaration;
-      }
-      if (deferred != null) {
-        // Coverage-ignore-block(suite): Not run.
-        _problemReporting.addProblem(
-            templateDeferredPrefixDuplicated.withArguments(name),
-            deferred.charOffset,
-            noLength,
-            _compilationUnit.fileUri,
-            context: [
-              templateDeferredPrefixDuplicatedCause
-                  .withArguments(name)
-                  .withLocation(
-                      _compilationUnit.fileUri, other!.charOffset, noLength)
-            ]);
-      }
-      existing.mergeScopes(declaration, _problemReporting, _nameSpace,
-          uriOffset: new UriOffset(_compilationUnit.fileUri, charOffset));
-      return existing;
-    } else if (isDuplicatedDeclaration(existing, declaration)) {
-      String fullName = name;
-      _problemReporting.addProblem(
-          templateDuplicatedDeclaration.withArguments(fullName),
-          charOffset,
-          fullName.length,
-          declaration.fileUri!,
-          context: <LocatedMessage>[
-            templateDuplicatedDeclarationCause
-                .withArguments(fullName)
-                .withLocation(
-                    existing!.fileUri!, existing.charOffset, fullName.length)
-          ]);
-    } else if (declaration.isExtension) {
-      // We add the extension declaration to the extension scope only if its
-      // name is unique. Only the first of duplicate extensions is accessible
-      // by name or by resolution and the remaining are dropped for the output.
-      _libraryTypeParameterScopeBuilder.extensions
-          .add(declaration as SourceExtensionBuilder);
-    } else if (declaration.isAugment) {
-      if (existing != null) {
-        if (declaration.isSetter) {
-          (_libraryTypeParameterScopeBuilder.setterAugmentations[name] ??= [])
-              .add(declaration);
-        } else {
-          (_libraryTypeParameterScopeBuilder.augmentations[name] ??= [])
-              .add(declaration);
-        }
-      } else {
-        // TODO(cstefantsova): Report an error.
-      }
-    } else if (declaration is PrefixBuilder) {
-      _prefixBuilders ??= <PrefixBuilder>[];
-      _prefixBuilders!.add(declaration);
-    }
-    return members[name] = declaration;
+    return _libraryNameSpaceBuilder.addBuilder(_parent, _problemReporting, name,
+        declaration, _compilationUnit.fileUri, charOffset);
   }
 
   void _addBuilderToDeclaration(
@@ -2973,16 +2860,7 @@
   List<MetadataBuilder>? get metadata => _metadata;
 
   @override
-  Iterable<Builder> get members =>
-      _libraryTypeParameterScopeBuilder.members.values;
-
-  @override
-  Iterable<Builder> get setters =>
-      _libraryTypeParameterScopeBuilder.setters.values;
-
-  @override
-  Iterable<ExtensionBuilder> get extensions =>
-      _libraryTypeParameterScopeBuilder.extensions;
+  Iterable<Builder> get builders => _libraryNameSpaceBuilder.builders;
 
   @override
   bool get isPart => _partOfName != null || _partOfUri != null;
@@ -2997,7 +2875,8 @@
   List<Part> get parts => _parts;
 
   @override
-  List<PrefixBuilder>? get prefixBuilders => _prefixBuilders;
+  List<PrefixBuilder>? get prefixBuilders =>
+      _libraryNameSpaceBuilder.prefixBuilders;
 
   @override
   void registerUnresolvedNamedTypes(List<NamedTypeBuilder> unboundTypes) {
diff --git a/pkg/front_end/lib/src/source/source_compilation_unit.dart b/pkg/front_end/lib/src/source/source_compilation_unit.dart
index 0e76a18..22c556c 100644
--- a/pkg/front_end/lib/src/source/source_compilation_unit.dart
+++ b/pkg/front_end/lib/src/source/source_compilation_unit.dart
@@ -83,7 +83,7 @@
   final bool isUnsupported;
 
   SourceCompilationUnitImpl(this._sourceLibraryBuilder,
-      TypeParameterScopeBuilder libraryTypeParameterScopeBuilder,
+      LibraryNameSpaceBuilder libraryNameSpaceBuilder,
       {required this.importUri,
       required this.fileUri,
       required Uri? packageUri,
@@ -107,10 +107,9 @@
         compilationUnit: this,
         augmentationRoot: _sourceLibraryBuilder,
         parent: _sourceLibraryBuilder,
-        libraryTypeParameterScopeBuilder: libraryTypeParameterScopeBuilder,
+        libraryNameSpaceBuilder: libraryNameSpaceBuilder,
         problemReporting: this,
         scope: _scope,
-        nameSpace: _nameSpace,
         libraryName: _libraryName,
         indexedLibrary: indexedLibrary,
         omittedTypeDeclarationBuilders: omittedTypeDeclarationBuilders);
@@ -1052,18 +1051,9 @@
       }
     }
 
-    for (Builder declaration in _builderFactoryResult.members) {
+    for (Builder declaration in _builderFactoryResult.builders) {
       computeDefaultValuesForDeclaration(declaration);
     }
-    for (Builder declaration in _builderFactoryResult.setters) {
-      computeDefaultValuesForDeclaration(declaration);
-    }
-    for (ExtensionBuilder declaration in _builderFactoryResult.extensions) {
-      if (declaration is SourceExtensionBuilder &&
-          declaration.isUnnamedExtension) {
-        computeDefaultValuesForDeclaration(declaration);
-      }
-    }
     return count;
   }
 
@@ -1162,7 +1152,7 @@
   int computeVariances() {
     int count = 0;
 
-    for (Builder? declaration in _builderFactoryResult.members) {
+    for (Builder? declaration in _builderFactoryResult.builders) {
       while (declaration != null) {
         if (declaration is TypeAliasBuilder &&
             declaration.typeVariablesCount > 0) {
diff --git a/pkg/front_end/lib/src/source/source_library_builder.dart b/pkg/front_end/lib/src/source/source_library_builder.dart
index 268a0e2..96c0057 100644
--- a/pkg/front_end/lib/src/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/source/source_library_builder.dart
@@ -219,8 +219,8 @@
                     : indexedLibrary?.library.reference)
           ..setLanguageVersion(packageLanguageVersion.version));
     LibraryName libraryName = new LibraryName(library.reference);
-    TypeParameterScopeBuilder libraryTypeParameterScopeBuilder =
-        new TypeParameterScopeBuilder();
+    LibraryNameSpaceBuilder libraryNameSpaceBuilder =
+        new LibraryNameSpaceBuilder();
     NameSpace? importNameSpace = new NameSpaceImpl();
     LookupScope importScope = new NameSpaceLookupScope(
         importNameSpace, ScopeKind.library, 'top',
@@ -228,7 +228,7 @@
     importScope = new FixedLookupScope(
         ScopeKind.typeParameters, 'omitted-types',
         getables: omittedTypes, parent: importScope);
-    NameSpace libraryNameSpace = libraryTypeParameterScopeBuilder.toNameSpace();
+    NameSpace libraryNameSpace = libraryNameSpaceBuilder.toNameSpace();
     NameSpace exportNameSpace = origin?.exportNameSpace ?? new NameSpaceImpl();
     return new SourceLibraryBuilder._(
         loader: loader,
@@ -237,7 +237,7 @@
         packageUri: packageUri,
         originImportUri: originImportUri,
         packageLanguageVersion: packageLanguageVersion,
-        libraryTypeParameterScopeBuilder: libraryTypeParameterScopeBuilder,
+        libraryNameSpaceBuilder: libraryNameSpaceBuilder,
         importNameSpace: importNameSpace,
         importScope: importScope,
         libraryNameSpace: libraryNameSpace,
@@ -251,9 +251,8 @@
         isAugmentation: isAugmentation,
         isPatch: isPatch,
         omittedTypes: omittedTypes,
-        augmentations: libraryTypeParameterScopeBuilder.augmentations,
-        setterAugmentations:
-            libraryTypeParameterScopeBuilder.setterAugmentations);
+        augmentations: libraryNameSpaceBuilder.augmentations,
+        setterAugmentations: libraryNameSpaceBuilder.setterAugmentations);
   }
 
   SourceLibraryBuilder._(
@@ -263,7 +262,7 @@
       required Uri? packageUri,
       required Uri originImportUri,
       required LanguageVersion packageLanguageVersion,
-      required TypeParameterScopeBuilder libraryTypeParameterScopeBuilder,
+      required LibraryNameSpaceBuilder libraryNameSpaceBuilder,
       required NameSpace importNameSpace,
       required LookupScope importScope,
       required NameSpace libraryNameSpace,
@@ -302,7 +301,7 @@
     _scope = new SourceLibraryBuilderScope(
         this, ScopeKind.typeParameters, 'library');
     compilationUnit = new SourceCompilationUnitImpl(
-        this, libraryTypeParameterScopeBuilder,
+        this, libraryNameSpaceBuilder,
         importUri: importUri,
         fileUri: fileUri,
         packageUri: _packageUri,
diff --git a/pkg/front_end/lib/src/source/type_parameter_scope_builder.dart b/pkg/front_end/lib/src/source/type_parameter_scope_builder.dart
index 3bd4e8a..3f944f3 100644
--- a/pkg/front_end/lib/src/source/type_parameter_scope_builder.dart
+++ b/pkg/front_end/lib/src/source/type_parameter_scope_builder.dart
@@ -6,37 +6,158 @@
 import '../base/name_space.dart';
 import '../base/problems.dart';
 import '../base/scope.dart';
+import '../base/uri_offset.dart';
 import '../builder/builder.dart';
 import '../builder/declaration_builders.dart';
 import '../builder/function_builder.dart';
 import '../builder/member_builder.dart';
+import '../builder/prefix_builder.dart';
 import '../builder/type_builder.dart';
 import 'name_scheme.dart';
+import 'source_extension_builder.dart';
 import 'source_field_builder.dart';
+import 'source_library_builder.dart';
 
-/// A builder object preparing for building declarations that can introduce type
-/// parameter and/or members.
-///
-/// Unlike [Scope], this scope is used during construction of builders to
-/// ensure types and members are added to and resolved in the correct location.
-class TypeParameterScopeBuilder {
-  final Map<String, Builder> members = {};
+class LibraryNameSpaceBuilder {
+  final Map<String, Builder> _members = {};
 
-  final Map<String, MemberBuilder> setters = {};
+  final Map<String, MemberBuilder> _setters = {};
 
-  final Set<ExtensionBuilder> extensions = {};
+  final Set<ExtensionBuilder> _extensions = {};
 
   final Map<String, List<Builder>> augmentations = {};
 
   final Map<String, List<Builder>> setterAugmentations = {};
 
-  NameSpace toNameSpace() {
-    return new NameSpaceImpl(
-        getables: members, setables: setters, extensions: extensions);
+  /// List of [PrefixBuilder]s for imports with prefixes.
+  List<PrefixBuilder>? _prefixBuilders;
+
+  late final NameSpace _nameSpace;
+
+  LibraryNameSpaceBuilder() {
+    _nameSpace = new NameSpaceImpl(
+        getables: _members, setables: _setters, extensions: _extensions);
   }
 
-  @override
-  String toString() => 'TypeParameterScopeBuilder(${hashCode})';
+  Iterable<Builder> get builders => [
+        ..._members.values,
+        ..._setters.values,
+        for (Builder builder in _extensions)
+          if (builder is SourceExtensionBuilder && builder.isUnnamedExtension)
+            builder
+      ];
+
+  Builder addBuilder(
+      SourceLibraryBuilder _parent,
+      ProblemReporting _problemReporting,
+      String name,
+      Builder declaration,
+      Uri fileUri,
+      int charOffset) {
+    if (declaration is SourceExtensionBuilder &&
+        declaration.isUnnamedExtension) {
+      declaration.parent = _parent;
+      _extensions.add(declaration);
+      return declaration;
+    }
+
+    if (declaration is MemberBuilder) {
+      declaration.parent = _parent;
+    } else if (declaration is TypeDeclarationBuilder) {
+      declaration.parent = _parent;
+    } else if (declaration is PrefixBuilder) {
+      assert(declaration.parent == _parent);
+    } else {
+      return unhandled(
+          "${declaration.runtimeType}", "addBuilder", charOffset, fileUri);
+    }
+
+    assert(
+        !(declaration is FunctionBuilder &&
+            (declaration.isConstructor || declaration.isFactory)),
+        // Coverage-ignore(suite): Not run.
+        "Unexpected constructor in library: $declaration.");
+
+    Map<String, Builder> members =
+        declaration.isSetter ? _setters : this._members;
+
+    Builder? existing = members[name];
+
+    if (existing == declaration) return declaration;
+
+    if (declaration.next != null && declaration.next != existing) {
+      unexpected(
+          "${declaration.next!.fileUri}@${declaration.next!.charOffset}",
+          "${existing?.fileUri}@${existing?.charOffset}",
+          declaration.charOffset,
+          declaration.fileUri);
+    }
+    declaration.next = existing;
+    if (declaration is PrefixBuilder && existing is PrefixBuilder) {
+      assert(existing.next is! PrefixBuilder);
+      Builder? deferred;
+      Builder? other;
+      if (declaration.deferred) {
+        deferred = declaration;
+        other = existing;
+      } else if (existing.deferred) {
+        deferred = existing;
+        other = declaration;
+      }
+      if (deferred != null) {
+        // Coverage-ignore-block(suite): Not run.
+        _problemReporting.addProblem(
+            templateDeferredPrefixDuplicated.withArguments(name),
+            deferred.charOffset,
+            noLength,
+            fileUri,
+            context: [
+              templateDeferredPrefixDuplicatedCause
+                  .withArguments(name)
+                  .withLocation(fileUri, other!.charOffset, noLength)
+            ]);
+      }
+      existing.mergeScopes(declaration, _problemReporting, _nameSpace,
+          uriOffset: new UriOffset(fileUri, charOffset));
+      return existing;
+    } else if (isDuplicatedDeclaration(existing, declaration)) {
+      String fullName = name;
+      _problemReporting.addProblem(
+          templateDuplicatedDeclaration.withArguments(fullName),
+          charOffset,
+          fullName.length,
+          declaration.fileUri!,
+          context: <LocatedMessage>[
+            templateDuplicatedDeclarationCause
+                .withArguments(fullName)
+                .withLocation(
+                    existing!.fileUri!, existing.charOffset, fullName.length)
+          ]);
+    } else if (declaration.isExtension) {
+      // We add the extension declaration to the extension scope only if its
+      // name is unique. Only the first of duplicate extensions is accessible
+      // by name or by resolution and the remaining are dropped for the output.
+      _extensions.add(declaration as SourceExtensionBuilder);
+    } else if (declaration.isAugment) {
+      if (existing != null) {
+        if (declaration.isSetter) {
+          (setterAugmentations[name] ??= []).add(declaration);
+        } else {
+          (augmentations[name] ??= []).add(declaration);
+        }
+      } else {
+        // TODO(cstefantsova): Report an error.
+      }
+    } else if (declaration is PrefixBuilder) {
+      _prefixBuilders ??= <PrefixBuilder>[];
+      _prefixBuilders!.add(declaration);
+    }
+    return members[name] = declaration;
+  }
+
+  List<PrefixBuilder>? get prefixBuilders => _prefixBuilders;
+
+  NameSpace toNameSpace() => _nameSpace;
 }
 
 class NominalParameterScope extends AbstractTypeParameterScope {