Introduce DartType super hierarchy based on Entity

pkg/compiler/lib/src/ssa/codegen.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.
 
 import 'dart:math' as math;
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
@@ skipping 2985 matching lines @@
         int index = 0;
         js.Expression result = backend.rtiEncoder.getTypeRepresentation(
             node.dartType, (TypeVariableType variable) => arguments[index++]);
         assert(index == node.inputs.length);
         push(result);
         return;
 
       case TypeInfoExpressionKind.INSTANCE:
         // We expect only flat types for the INSTANCE representation.
         assert(
-            node.dartType == (node.dartType.element as ClassElement).thisType);
+            node.dartType == (node.dartType as InterfaceType).element.thisType);
         registry.registerInstantiatedClass(commonElements.listClass);
         push(new js.ArrayInitializer(arguments)
             .withSourceInformation(node.sourceInformation));
     }
   }
 
   bool typeVariableAccessNeedsSubstitution(
       TypeVariableElement element, TypeMask receiverMask) {
     ClassElement cls = element.enclosingClass;
 
@@ skipping 47 matching lines @@
           [backend.emitter.staticFunctionAccess(helperElement), pop()]));
     }
   }
 
   void visitInterfaceType(HInterfaceType node) {
     List<js.Expression> typeArguments = <js.Expression>[];
     for (HInstruction type in node.inputs) {
       use(type);
       typeArguments.add(pop());
     }
-
-    ClassElement cls = node.dartType.element;
+    InterfaceType type = node.dartType;
+    ClassElement cls = type.element;
     var arguments = [backend.emitter.typeAccess(cls)];
     if (!typeArguments.isEmpty) {
       arguments.add(new js.ArrayInitializer(typeArguments));
     }
     push(js.js('#(#)',
         [accessHelper('buildInterfaceType', arguments.length), arguments]));
   }
 
   void visitVoidType(HVoidType node) {
     push(js.js('#()', accessHelper('getVoidRuntimeType')));
@@ skipping 12 matching lines @@
     registry.registerStaticUse(new StaticUse.staticInvoke(
         helper, new CallStructure.unnamed(argumentCount)));
     return backend.emitter.staticFunctionAccess(helper);
   }
 
   @override
   void visitRef(HRef node) {
     visit(node.value);
   }
 }