Use entities in CommonElements interface.

tests/compiler/dart2js/type_combination_test.dart

Index: tests/compiler/dart2js/type_combination_test.dart
diff --git a/tests/compiler/dart2js/type_combination_test.dart b/tests/compiler/dart2js/type_combination_test.dart
index 153ef44407fa467954bda02087085df14827c87f..f54364e74d6f0a5c307786ee2cebf4404b64de32 100644
--- a/tests/compiler/dart2js/type_combination_test.dart
+++ b/tests/compiler/dart2js/type_combination_test.dart
@@ -768,7 +768,8 @@ void main() {
 
     // Grab hold of a supertype for String so we can produce potential
     // string types.
-    patternClass = closedWorld.commonElements.coreLibrary.find('Pattern');
+    LibraryElement coreLibrary = closedWorld.commonElements.coreLibrary;
+    patternClass = coreLibrary.find('Pattern');
 
     nonPrimitive1 = new TypeMask.nonNullSubtype(
         closedWorld.commonElements.mapClass, closedWorld);