Re-apply "support for typedef in checked mode" and fix the checked mode failures on drt by not shar…

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(revision 16252)
+++ sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(working copy)
@@ -1101,7 +1101,7 @@
       return const SourceString('numTypeCheck');
     } else if (element == compiler.boolClass) {
       return const SourceString('boolTypeCheck');
-    } else if (element == compiler.functionClass || element.isTypedef()) {
+    } else if (element == compiler.functionClass) {
       return const SourceString('functionTypeCheck');
     } else if (element == compiler.intClass) {
       return const SourceString('intTypeCheck');