Add names to nested declaration to ease debugging.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index a21b00c8c8d86f4abd710dbd7bb3bd19c64492a6..1583ab9e0f10b238dcf714d9ae954b1ec53aaee2 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -120,7 +120,12 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
     currentDeclaration = currentDeclaration.createNested(name, hasMembers);
   }
 
-  DeclarationBuilder<T> endNestedDeclaration() {
+  DeclarationBuilder<T> endNestedDeclaration(String name) {
+    assert(
+        (name?.startsWith(currentDeclaration.name) ??
+                (name == currentDeclaration.name)) ||
+            currentDeclaration.name == "operator",
+        "${name} != ${currentDeclaration.name}");
     DeclarationBuilder<T> previous = currentDeclaration;
     currentDeclaration = currentDeclaration.parent;
     return previous;