Fix handling of boxed loop variable in CPS IR

pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index 529cdd4f98e3e33ac5a0c5dd9c1c17aa5fc32ee4..02c877c9fc82b07e38d7e7f7eb5617399165249b 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -2535,6 +2535,7 @@ class IrBuilder {
     newBox.useElementAsHint(scope.box);
     for (VariableElement loopVar in scope.boxedLoopVariables) {
       ClosureLocation location = scope.capturedVariables[loopVar];
+      if (location == null) continue;

[Siggi Cherem (dart-lang), 2015/10/23 01:39:58]

I'm not certain that this is the right fix. What I'm seeing is that
boxedLoopVariables are also in capturedVariables if they are updated outside of
the loop-update section. In that case, we don't include it in the
capturedVariables map.

The example from the test where this happens looks like this:
    main() {
      var f;
      for (int n = 0; n < 10; n++) {
        int k = n;
        f = () => 'k $k n $n';
        k++;
      }
      print(f());
    }

`n` is in boxedLoopedVariables, but only `k` is in capturedVariables.

Rather than skipping it here, should this be handled differently by the
ClosureScope in `closure.dart`?

[asgerf, 2015/10/23 09:37:30]

'n' is captured but not boxed, so it's absurd that it should be in
boxedLoopVariables and not capturedVariables.

I've proposed a fix in closure.dart here:
https://codereview.chromium.org/1409933004/

       ir.Primitive value = addPrimitive(new ir.GetField(box, location.field));
       addPrimitive(new ir.SetField(newBox, location.field, value));
     }