Don't try to box immutable objects.

lib/compiler/implementation/ssa/closure.dart

 // Copyright (c) 2012, 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.
 
 class ClosureFieldElement extends Element {
   ClosureFieldElement(SourceString name, ClassElement enclosing)
       : super(name, ElementKind.FIELD, enclosing);
 
   bool isInstanceMember() => true;
   bool isAssignable() => false;
@@ skipping 222 matching lines @@
   }
 
   // If variables that are declared in the [node] scope are captured and need
   // to be boxed create a box-element and update the [capturingScopes] in the
   // current [closureData].
   // The boxed variables are updated in the [capturedVariableMapping].
   void attachCapturedScopeVariables(Node node) {
     Element box = null;
     Map<Element, Element> scopeMapping = new Map<Element, Element>();
     for (Element element in scopeVariables) {
+      // No need to box non-assignable elements.
+      if (!element.isAssignable()) continue;
       if (capturedVariableMapping.containsKey(element)) {
         if (box == null) {
           // TODO(floitsch): construct better box names.
           SourceString boxName = new SourceString("box${boxCounter++}");
           box = new Element(boxName,
                             ElementKind.VARIABLE,
                             currentFunctionElement);
         }
         // TODO(floitsch): construct better boxed names.
         String elementName = element.name.slowToString();
@@ skipping 94 matching lines @@
           thisEnclosingElement = body.constructor;
         }
         thisElement = new Element(const SourceString("this"),
                                   ElementKind.PARAMETER,
                                   thisEnclosingElement);
       }
       closureData = new ClosureData(null, null, null, thisElement);
     }
     scopeVariables = new List<Element>();
 
-    // TODO(floitsch): a named function is visible from inside itself. Add
-    // the element to the block.
-
     // We have to declare the implicit 'this' parameter.
     if (!insideClosure && closureData.thisElement !== null) {
       declareLocal(closureData.thisElement);
     }
     // If we are inside a named closure we have to declare ourselve. For
     // simplicity we declare the local even if the closure does not have a name
     // It will simply not be used.
     if (insideClosure) {
       declareLocal(element);
     }
@@ skipping 38 matching lines @@
     declareLocal(elements[node]);
   }
 
   visitTryStatement(TryStatement node) {
     // TODO(ngeoffray): implement finer grain state.
     inTryCatchOrFinally = true;
     node.visitChildren(this);
     inTryCatchOrFinally = false;
   }
 }