Introduce DartType super hierarchy based on Entity

pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart

 // Copyright (c) 2013, 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 '../compiler.dart' show Compiler;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../js/js.dart' as jsAst;
 import '../js/js.dart' show js;
 import '../ssa/codegen.dart' show SsaCodeGenerator;
@@ skipping 71 matching lines @@
   ];
 }
 
 class MalformedCheckedModeHelper extends CheckedModeHelper {
   const MalformedCheckedModeHelper(String name) : super(name);
 
   CallStructure get callStructure => CallStructure.TWO_ARGS;
 
   void generateAdditionalArguments(SsaCodeGenerator codegen,
       HTypeConversion node, List<jsAst.Expression> arguments) {
-    ErroneousElement element = node.typeExpression.element;
+    MalformedType type = node.typeExpression;
+    ErroneousElement element = type.element;
     arguments.add(js.escapedString(element.message));
   }
 }
 
 class PropertyCheckedModeHelper extends CheckedModeHelper {
   const PropertyCheckedModeHelper(String name) : super(name);
 
   CallStructure get callStructure => CallStructure.TWO_ARGS;
 
   void generateAdditionalArguments(SsaCodeGenerator codegen,
@@ skipping 27 matching lines @@
     DartType type = node.typeExpression;
     Element element = type.element;
     jsAst.Name isField = codegen.backend.namer.operatorIs(element);
     arguments.add(js.quoteName(isField));
     codegen.use(node.typeRepresentation);
     arguments.add(codegen.pop());
     jsAst.Name asField = codegen.backend.namer.substitutionName(element);
     arguments.add(js.quoteName(asField));
   }
 }