Record static types in the desugaring of null-aware property gets.

pkg/front_end/testcases/inference/null_aware_property_get.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   field core::int x = null;
   constructor •() → void
     : super core::Object::•()
     ;
 }
 static method f(self::C c) → void {
-  core::int x = let final dynamic #t1 = c in #t1.==(null) ?{dynamic} null : #t1.{self::C::x};
+  core::int x = let final self::C #t1 = c in #t1.==(null) ?{core::int} null : #t1.{self::C::x};
 }
 static method main() → dynamic {}