Introduce Substitution class and Supertype class.

test/type_substitution_identity_test.dart

Index: test/type_substitution_identity_test.dart
diff --git a/test/type_substitution_identity_test.dart b/test/type_substitution_identity_test.dart
index d5f76a7af185aa1389bdcf96ab7414e487ad42a1..d07da2adc3cfc5c3318db0508e484a8184c35ccf 100644
--- a/test/type_substitution_identity_test.dart
+++ b/test/type_substitution_identity_test.dart
@@ -13,7 +13,7 @@ checkType(DartType type) {
   if (!identical(type, other)) {
     fail('Identity substitution test failed for $type');
   }
-  other = substituteBounds(type, map, map);
+  other = Substitution.fromUpperAndLowerBounds(map, map).substituteType(type);
   if (!identical(type, other)) {
     fail('Identity bounded substitution test failed for $type');
   }