Only set types of conditional expressions in strong mode.

pkg/front_end/testcases/rasta/duplicated_mixin.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Mixin extends core::Object {
   field dynamic field = null;
   constructor •() → void
     : super core::Object::•()
     ;
 }
-abstract class Object&Mixin = core::Object with self::Mixin {
-}
-abstract class Object&Mixin&Mixin = self::Object&Mixin with self::Mixin {
-}
-class A extends self::Object&Mixin&Mixin {
+abstract class Object&Mixin extends core::Object implements self::Mixin {
+  field dynamic field = null;
   constructor •() → void
     : super core::Object::•()
     ;
 }
+abstract class Object&Mixin&Mixin extends self::Object&Mixin implements self::Mixin {
+  field dynamic field = null;
+  constructor •() → void
+    : super self::Object&Mixin::•()
+    ;
+}
+class A extends self::Object&Mixin&Mixin {
+  constructor •() → void
+    : super self::Object&Mixin&Mixin::•()
+    ;
+}