Introduce DartType super hierarchy based on Entity

pkg/compiler/lib/src/dart_types.dart

Index: pkg/compiler/lib/src/dart_types.dart
diff --git a/pkg/compiler/lib/src/dart_types.dart b/pkg/compiler/lib/src/dart_types.dart
index b151ed99b75193dfdc0ca3a52022f190c841e35f..fc5b81424c6a123251795a5ff00296ffef4d23e2 100644
--- a/pkg/compiler/lib/src/dart_types.dart
+++ b/pkg/compiler/lib/src/dart_types.dart
@@ -10,6 +10,7 @@ import 'common/resolution.dart' show Resolution;
 import 'common.dart';
 import 'core_types.dart';
 import 'elements/elements.dart';
+import 'elements/types.dart' as types;
 import 'elements/modelx.dart' show TypeDeclarationElementX;
 import 'ordered_typeset.dart' show OrderedTypeSet;
 import 'util/util.dart' show equalElements;
@@ -24,7 +25,7 @@ enum TypeKind {
   VOID,
 }
 
-abstract class DartType {
+abstract class DartType implements types.DartType {
   String get name;
 
   TypeKind get kind;
@@ -196,7 +197,7 @@ abstract class DartType {
  * [: String :] because we must substitute [: String :] for the
  * the type variable [: T :].
  */
-class TypeVariableType extends DartType {
+class TypeVariableType extends DartType implements types.TypeVariableType {
   final TypeVariableElement element;
 
   TypeVariableType(this.element);
@@ -261,7 +262,7 @@ class MethodTypeVariableType extends TypeVariableType {
   get containsMethodTypeVariableType => true;
 }
 
-class VoidType extends DartType {
+class VoidType extends DartType implements types.VoidType {
   const VoidType();
 
   TypeKind get kind => TypeKind.VOID;
@@ -467,7 +468,7 @@ abstract class GenericType extends DartType {
   }
 }
 
-class InterfaceType extends GenericType {
+class InterfaceType extends GenericType implements types.InterfaceType {
   int _hashCode;
 
   InterfaceType(ClassElement element,
@@ -576,7 +577,7 @@ class BadTypedefType extends TypedefType {
   }
 }
 
-class FunctionType extends DartType {
+class FunctionType extends DartType implements types.FunctionType {
   final FunctionTypedElement element;
   final DartType returnType;
   final List<DartType> parameterTypes;
@@ -859,7 +860,7 @@ class TypedefType extends GenericType {
 /**
  * Special type for the `dynamic` type.
  */
-class DynamicType extends DartType {
+class DynamicType extends DartType implements types.DynamicType {
   const DynamicType();
 
   Element get element => null;