Add support for converted closures with explicit contexts to VM

pkg/kernel/testcases/closures/contexts_in_field_initializers.dart.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class X extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class A extends core::Object {
   field self::X x;
   constructor •(self::X x) → void
     : self::A::x = x, super core::Object::•() {}
 }
 static method main() → dynamic {
   self::A a = new self::A::•(new self::X::•());
 }