Introduce DartType super hierarchy based on Entity

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

 // Copyright (c) 2012, 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 elements;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution;
 import '../compiler.dart' show Compiler;
 import '../constants/constructors.dart';
@@ skipping 1662 matching lines @@
 
   // TODO(kasperl): Try to get rid of these.
   void set isBreakTarget(bool value);
   void set isContinueTarget(bool value);
 
   LabelDefinition addLabel(Label label, String labelName);
 }
 
 /// The [Element] for a type variable declaration on a generic class or typedef.
 abstract class TypeVariableElement extends Element
-    implements AstElement, TypedElement {
+    implements AstElement, TypedElement, TypeVariableEntity {
   /// The name of this type variable, taking privacy into account.
   Name get memberName;
 
   /// Use [typeDeclaration] instead.
   @deprecated
   get enclosingElement;
 
   /// The class, typedef, function, method, or function typed parameter on
   /// which this type variable is defined.
   GenericElement get typeDeclaration;
@@ skipping 240 matching lines @@
   /// by a field.
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }