Fix some issues after introducing NamedType.

testcases/type-propagation/named_parameters.baseline.txt

 library;
 import self as self;
 import "dart:core" as core;
 
 class Superclass extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
   method foo({dynamic/Null alpha, dynamic/Null beta}) → dynamic/Null {}
   method bar({dynamic/Null alpha, dynamic/Null beta}) → dynamic/Null {}
-  method namedCallback(({alpha: dynamic, beta: dynamic}) → dynamic/Nothing callback) → dynamic/Null {}
+  method namedCallback(({alpha: core::String, beta: core::int}) → dynamic/Nothing callback) → dynamic/Null {
+    callback.call(alpha: "one", beta: 2);
+    callback.call(beta: 1, alpha: "two");
+  }
 }
 class Subclass extends self::Superclass {
   constructor •() → void
     : super self::Superclass::•()
     ;
   method foo({dynamic/core::int* {null,int} alpha, dynamic/core::int* {null,int} beta}) → dynamic/Null {}
   method bar({dynamic/Null alpha, dynamic/Null beta}) → dynamic/Null {}
-  method namedCallback(({alpha: dynamic, beta: dynamic}) → dynamic/Nothing callback) → dynamic/Null {}
+  method namedCallback(({alpha: core::String, beta: core::int}) → dynamic/Nothing callback) → dynamic/Null {}
 }
 static method topLevelNamed(dynamic/core::int* {int} beta, dynamic/core::int* {int} alpha, {dynamic/core::int* {null,int} delta, dynamic/core::int* {null,int} gamma}) → dynamic/Null {}
 static method topLevelOptional(dynamic/Nothing beta, dynamic/Nothing alpha, [dynamic/Null gamma, dynamic/Null delta]) → dynamic/Null {}
 static method main() → dynamic/Null {
   new self::Subclass::•().foo(beta: 1, alpha: 2);
   new self::Subclass::•().foo(alpha: 1, beta: 2);
   self::topLevelNamed(1, 2, gamma: 3, delta: 4);
   self::topLevelNamed(1, 2, delta: 3, gamma: 4);
 }