Revert "Normalize type masks to use the least upper instantiated subclass/type."

tests/compiler/dart2js/mock_libraries.dart

Index: tests/compiler/dart2js/mock_libraries.dart
diff --git a/tests/compiler/dart2js/mock_libraries.dart b/tests/compiler/dart2js/mock_libraries.dart
index 9b89db0a89da4479718895d685ed38fb722ab755..e73586c0cc04b438e237dfc44677548a7323f2fd 100644
--- a/tests/compiler/dart2js/mock_libraries.dart
+++ b/tests/compiler/dart2js/mock_libraries.dart
@@ -63,7 +63,7 @@ const Map<String, String> DEFAULT_CORE_LIBRARY = const <String, String>{
           E get current => null;
       }''',
   'LinkedHashMap': r'''
-      class LinkedHashMap<K, V> implements Map<K, V> {
+      class LinkedHashMap {
         factory LinkedHashMap._empty() => null;
         factory LinkedHashMap._literal(elements) => null;
         static _makeEmpty() => null;