Introduce DartType super hierarchy based on Entity

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

Index: pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
index 38e00709a4fca73bf70be6803cebaae88b16f010..57487179f081d74d350f0662d6a136d2955eeebd 100644
--- a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
@@ -89,7 +89,8 @@ class MalformedCheckedModeHelper extends CheckedModeHelper {
 
   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));
   }
 }