Reduce use of Compiler.closedWorld

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index fb4f1537c442a8ad10d6649e9aedce8e81001b95..764341ce993875a0a9399ce07be909c11eba3568 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -397,6 +397,7 @@ abstract class BackendClasses {
   ClassElement get numImplementation;
   ClassElement get stringImplementation;
   ClassElement get listImplementation;
+  ClassElement get mutableListImplementation;
   ClassElement get growableListImplementation;
   ClassElement get fixedListImplementation;
   ClassElement get constListImplementation;
@@ -412,4 +413,8 @@ abstract class BackendClasses {
   ClassElement get syncStarIterableImplementation;
   ClassElement get asyncFutureImplementation;
   ClassElement get asyncStarStreamImplementation;
+  ClassElement get indexableImplementation;
+  ClassElement get mutableIndexableImplementation;
+
+  bool isDefaultEqualityImplementation(Element element);
 }