Fix hints reported by analyzer.

pkg/compiler/lib/src/js_backend/minify_namer.dart

Index: pkg/compiler/lib/src/js_backend/minify_namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/minify_namer.dart b/pkg/compiler/lib/src/js_backend/minify_namer.dart
index f0fdcb872fd88b4f828890ed2e68e4301ef04cf3..2871687c17c53e6c817ba3b65e2b3f808a121cfe 100644
--- a/pkg/compiler/lib/src/js_backend/minify_namer.dart
+++ b/pkg/compiler/lib/src/js_backend/minify_namer.dart
@@ -317,25 +317,20 @@ class _ConstructorBodyNamingScope {
 
   int get numberOfConstructors => _constructors.length;
 
-  _ConstructorBodyNamingScope _superScope;
-
   _ConstructorBodyNamingScope.rootScope(ClassElement cls)
-      : _superScope = null,
-        _startIndex = 0,
+      : _startIndex = 0,
         _constructors = cls.constructors.toList(growable: false);
 
   _ConstructorBodyNamingScope.forClass(
       ClassElement cls, _ConstructorBodyNamingScope superScope)
-      : _superScope = superScope,
-        _startIndex = superScope._startIndex + superScope.numberOfConstructors,
+      : _startIndex = superScope._startIndex + superScope.numberOfConstructors,
         _constructors = cls.constructors.toList(growable: false);
 
   // Mixin Applications have constructors but we never generate code for them,
   // so they do not count in the inheritance chain.
   _ConstructorBodyNamingScope.forMixinApplication(
       ClassElement cls, _ConstructorBodyNamingScope superScope)
-      : _superScope = superScope,
-        _startIndex = superScope._startIndex + superScope.numberOfConstructors,
+      : _startIndex = superScope._startIndex + superScope.numberOfConstructors,
         _constructors = const [];
 
   factory _ConstructorBodyNamingScope(ClassElement cls,