Introduce Substitution class and Supertype class.

test/type_parser.dart

Index: test/type_parser.dart
diff --git a/test/type_parser.dart b/test/type_parser.dart
index b435554bbf04dd16fb10648f2e544ff1714f82dc..0dc1875bcd22ceb22c643c026ec40f2f0725d7e8 100644
--- a/test/type_parser.dart
+++ b/test/type_parser.dart
@@ -295,6 +295,11 @@ class LazyTypeEnvironment {
 
   DartType parse(String type) => parseDartType(type, lookup);
 
+  Supertype parseSuper(String type) {
+    InterfaceType interfaceType = parse(type);
+    return new Supertype(interfaceType.classNode, interfaceType.typeArguments);
+  }
+
   DartType parseFresh(String type) {
     clearTypeParameters();
     return parse(type);