[kernel] Support for top-level generic functions.

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

 library;
 import self as self;
 
 static method main() → dynamic {
   dynamic closure;
   {
-    Vector #context = MakeVector(2);
-    #context[1] = 0;
+    Vector #context = MakeVector(3);
+    #context[2] = 0;
     dynamic fn = MakeClosure<() → dynamic>(self::closure#main#function, #context);
-    for (; (#context[1]).<(3); #context = CopyVector(#context), #context[1] = (#context[1]).+(1)) {
-      #context[1] = (#context[1]).+(1);
+    for (; (#context[2]).<(3); #context = CopyVector(#context), #context[2] = (#context[2]).+(1)) {
+      #context[2] = (#context[2]).+(1);
       closure = fn;
     }
   }
   dynamic x = closure.call();
   if(!x.==(1)) {
     throw "Expected 1, but got ${x}.";
   }
 }
 static method closure#main#function(Vector #contextParameter) → dynamic {
-  return #contextParameter[1];
+  return #contextParameter[2];
 }