Add a TypeInformation abstraction in the inferrer, that holds information per element, to get rid o…

tests/compiler/dart2js/field_type_simple_inferer_test.dart

Index: tests/compiler/dart2js/field_type_simple_inferer_test.dart
===================================================================
--- tests/compiler/dart2js/field_type_simple_inferer_test.dart	(revision 24382)
+++ tests/compiler/dart2js/field_type_simple_inferer_test.dart	(working copy)
@@ -453,7 +453,7 @@
       (inferrer, field) {
         TypeMask type = f(inferrer);
         TypeMask inferredType =
-            inferrer.internal.typeOf[field].simplify(inferrer.compiler);
+            inferrer.getTypeOfElement(field).simplify(inferrer.compiler);
         Expect.equals(type, inferredType, name);
     });
   });