Refactor ConstantSystem

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

Index: pkg/compiler/lib/src/js_backend/backend_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_helpers.dart b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
index c6f9678a5c3d4e2131a101003b49afe9d571a395..5a086225bef51d4547c5e8ef93d827b2b5e1bc9b 100644
--- a/pkg/compiler/lib/src/js_backend/backend_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
@@ -441,6 +441,14 @@ class BackendHelpers {
   ClassEntity get symbolImplementationClass =>
       _symbolImplementationClass ??= _findClass(internalLibrary, 'Symbol');
 
+  FieldEntity _symbolImplementationField;
+
+  /// Returns the field that holds the internal name in the implementation class
+  /// for `Symbol`.
+  FieldEntity get symbolImplementationField => _symbolImplementationField ??=
+      _env.lookupClassMember(symbolImplementationClass, '_name',
+          required: true);
+
   ConstructorEntity _symbolValidatedConstructor;
 
   bool isSymbolValidatedConstructor(ConstructorEntity element) {