Introduce DartType super hierarchy based on Entity

pkg/compiler/lib/src/elements/entities.dart

Index: pkg/compiler/lib/src/elements/entities.dart
diff --git a/pkg/compiler/lib/src/elements/entities.dart b/pkg/compiler/lib/src/elements/entities.dart
index ce1fc567de623549a26b07ae6e1611e86b36bf92..f46c790603819198d8bb5e3677cff4f72fd4af02 100644
--- a/pkg/compiler/lib/src/elements/entities.dart
+++ b/pkg/compiler/lib/src/elements/entities.dart
@@ -16,6 +16,11 @@ abstract class ClassEntity extends Entity {
       {bool includeSuperAndInjectedMembers: false});
 }
 
+abstract class TypeVariableEntity extends Entity {
+  Entity get typeDeclaration;
+  int get index;
+}
+
 /// Stripped down super interface for member like entities, that is,
 /// constructors, methods, fields etc.
 ///