Only set types of conditional expressions in strong mode.

pkg/front_end/testcases/void-methods.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   field core::List<dynamic> list = <dynamic>[1, 2, 3];
   constructor •() → void
     : super core::Object::•()
     ;
   set first(dynamic x) → dynamic
-    return let final dynamic #t1 = this.list in let final dynamic #t2 = 0 in let final dynamic #t3 = x in let final dynamic #t4 = #t1.[]=(#t2, #t3) in #t3;
+    this.list.[]=(0, x);
   operator []=(dynamic x, dynamic y) → dynamic
-    return let final dynamic #t5 = this.list in let final dynamic #t6 = x in let final dynamic #t7 = y in let final dynamic #t8 = #t5.[]=(#t6, #t7) in #t7;
+    this.list.[]=(x, y);
   method clear() → void
     return this.list.clear();
 }
 static method main() → dynamic {
   new self::Foo::•().first = 4;
   new self::Foo::•().[]=(3, 4);
   new self::Foo::•().clear();
 }