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

tests/compiler/dart2js/call_site_simple_type_inferer_test.dart

Index: tests/compiler/dart2js/call_site_simple_type_inferer_test.dart
===================================================================
--- tests/compiler/dart2js/call_site_simple_type_inferer_test.dart	(revision 24382)
+++ tests/compiler/dart2js/call_site_simple_type_inferer_test.dart	(working copy)
@@ -222,7 +222,7 @@
       int index = 0;
       signature.forEachParameter((Element element) {
         Expect.equals(expectedTypes[index++],
-            inferrer.internal.typeOf[element].simplify(inferrer.compiler));
+            inferrer.getTypeOfElement(element).simplify(inferrer.compiler));
       });
       Expect.equals(index, expectedTypes.length);
   });