Fix handling of boxed loop variable in CPS IR

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.ir_builder;
 
 import '../closure.dart' hide ClosureScope;
 import '../common.dart';
 import '../common/names.dart' show
     Names,
@@ skipping 2517 matching lines @@
                            List<LocalElement> loopVariables) {
     if (scope == null) return;
     // If there are no boxed loop variables, then the box is created inside the
     // body, so there is no need to explicitly renew it.
     if (scope.boxedLoopVariables.isEmpty) return;
     ir.Primitive box = environment.lookup(scope.box);
     ir.Primitive newBox = addPrimitive(new ir.CreateBox());
     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));
     }
     environment.update(scope.box, newBox);
   }
 
   /// Creates an access to the receiver from the current (or enclosing) method.
   ///
   /// If inside a closure class, [buildThis] will redirect access through
   /// closure fields in order to access the receiver from the enclosing method.
@@ skipping 338 matching lines @@
 }
 
 class SwitchCaseInfo {
   final List<ir.Primitive> constants = <ir.Primitive>[];
   final SubbuildFunction buildBody;
 
   SwitchCaseInfo(this.buildBody);
 
   void addConstant(ir.Primitive constant) => constants.add(constant);
 }