Fix several bugs in closure conversion.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 static const field dynamic numbers = const <core::int>[0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
 static method main() → dynamic {
   dynamic closures = <dynamic>[];
+  final Vector #context = MakeVector(3);
+  #context[2] = 0;
   for (core::int i in self::numbers) {
     final Vector #context = MakeVector(3);
+    #context[1] = #context;
     #context[2] = i;
     {
       closures.add(MakeClosure<() → dynamic>(self::closure#main#function, #context));
     }
   }
   core::int sum = 0;
   for (core::Function f in closures) {
     sum = sum.+(f.call());
   }
   core::int expectedSum = self::numbers.length.-(1).*(self::numbers.length).~/(2);
   if(!expectedSum.==(sum)) {
     throw core::Exception::•("Unexpected sum = ${sum} != ${expectedSum}");
   }
 }
 static method closure#main#function(Vector #contextParameter) → dynamic {
-  return #contextParameter[2];
+  return (#contextParameter[2]).+(#contextParameter[1][2]);
 }