Add support for converted closures with explicit contexts to VM

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   field dynamic x = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic {
     final Vector #context = MakeVector(2);
     #context[1] = this;
     return MakeClosure<(dynamic) → dynamic>(self::closure#C#m#function, #context);
   }
   method f() → dynamic {
     final Vector #context = MakeVector(2);
     #context[1] = this;
-    return MakeClosure<() → () → dynamic>(self::closure#C#f#function, #context);
+    return MakeClosure<() → dynamic>(self::closure#C#f#function, #context);
   }
 }
 static method main() → dynamic {
   self::C c = new self::C::•();
   c.x = 41;
   c.m().call(42);
   if(!42.==(c.x))
     throw "Unexpected value in c.x: ${c.x}";
   dynamic result = c.f().call().call();
   if(!42.==(result))
     throw "Unexpected value from c.f()()(): ${result}";
 }
 static method closure#C#m#function(Vector #contextParameter, dynamic v) → dynamic {
   return (#contextParameter[1]).x = v;
 }
 static method closure#C#f#function#function(Vector #contextParameter) → dynamic {
   return (#contextParameter[1]).x;
 }
-static method closure#C#f#function(Vector #contextParameter) → () → dynamic {
+static method closure#C#f#function(Vector #contextParameter) → dynamic {
   return MakeClosure<() → dynamic>(self::closure#C#f#function#function, #contextParameter);
 }