Further reduce use of Element in codegen.

pkg/compiler/lib/src/core_types.dart

Index: pkg/compiler/lib/src/core_types.dart
diff --git a/pkg/compiler/lib/src/core_types.dart b/pkg/compiler/lib/src/core_types.dart
index 6f70cbb8bc078644ec69f1d09f5b4ab1d097095b..6f2f59fbd8f2642efb4db50551e5415b1a237988 100644
--- a/pkg/compiler/lib/src/core_types.dart
+++ b/pkg/compiler/lib/src/core_types.dart
@@ -199,4 +199,13 @@ abstract class CommonElements {
   ///
   /// If no type argument is provided, the canonical raw type is returned.
   InterfaceType streamType([DartType elementType]);
+
+  /// Returns `true` if [element] is a superclass of `String` or `num`.
+  bool isNumberOrStringSupertype(ClassElement element);
+
+  /// Returns `true` if [element] is a superclass of `String`.
+  bool isStringOnlySupertype(ClassElement element);
+
+  /// Returns `true` if [element] is a superclass of `List`.
+  bool isListSupertype(ClassElement element);
 }