Don't try to box immutable objects.

lib/compiler/implementation/ssa/closure.dart

Index: lib/compiler/implementation/ssa/closure.dart
diff --git a/lib/compiler/implementation/ssa/closure.dart b/lib/compiler/implementation/ssa/closure.dart
index ab2d6a48c541bdbf94572232acce6b9d99a23635..7d90b58eec68b241cc4e0c75f09aa9b7ac7eaccb 100644
--- a/lib/compiler/implementation/ssa/closure.dart
+++ b/lib/compiler/implementation/ssa/closure.dart
@@ -240,6 +240,8 @@ class ClosureTranslator extends AbstractVisitor {
     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.
@@ -354,9 +356,6 @@ class ClosureTranslator extends AbstractVisitor {
     }
     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);