Introduce DartType super hierarchy based on Entity

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library entities;
 
 import 'elements.dart' show Entity;
 
 /// Stripped down super interface for class like entities.
 ///
 /// Currently only [ClassElement] but later also kernel based Dart classes
 /// and/or Dart-in-JS classes.
 abstract class ClassEntity extends Entity {
   bool get isClosure;
   void forEachInstanceField(f(ClassEntity cls, FieldEntity field),
       {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.
 ///
 /// Currently only [MemberElement] but later also kernel based Dart members
 /// and/or Dart-in-JS properties.
 abstract class MemberEntity extends Entity {
   bool get isField;
   bool get isFunction;
   bool get isGetter;
   bool get isSetter;
   bool get isAssignable;
   ClassEntity get enclosingClass;
 }
 
 /// Stripped down super interface for field like entities.
 ///
 /// Currently only [FieldElement] but later also kernel based Dart fields
 /// and/or Dart-in-JS field-like properties.
 abstract class FieldEntity extends MemberEntity {}
 
 /// Stripped down super interface for function like entities.
 ///
 /// Currently only [FieldElement] but later also kernel based Dart constructors
 /// and methods and/or Dart-in-JS function-like properties.
 abstract class FunctionEntity extends MemberEntity {}