Only set types of conditional expressions in strong mode.

pkg/front_end/testcases/override.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
 }
 class Bar extends self::Foo {
@@ skipping 11 matching lines @@
 }
 class Sub extends self::Base {
   constructor •() → void
     : super self::Base::•()
     ;
   method method() → self::Foo {
     return new self::Bar::•();
   }
 }
 static method main(core::List<core::String> args) → dynamic {
-  dynamic object = args.length.==(0) ?{self::Base} new self::Base::•() : new self::Sub::•();
+  dynamic object = args.length.==(0) ? new self::Base::•() : new self::Sub::•();
   dynamic a = object.method();
   core::print(a);
 }