Do not use the namer for naming closure elements.

pkg/compiler/lib/src/js_backend/namer.dart

Index: pkg/compiler/lib/src/js_backend/namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/namer.dart b/pkg/compiler/lib/src/js_backend/namer.dart
index 9308dc16b2ef2f0271c0409e0f98c54fe9621686..1fd8d6086b69dba429a982e29b7eef62e07a6195 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -101,7 +101,7 @@ part of js_backend;
  * For local variables, the [Namer] only provides *proposed names*. These names
  * must be disambiguated elsewhere.
  */
-class Namer implements ClosureNamer {
+class Namer {
 
   static const List<String> javaScriptKeywords = const <String>[
     // These are current keywords.
@@ -663,14 +663,24 @@ class Namer implements ClosureNamer {
     ClassElement enclosingClass = element.enclosingClass;
 
     if (element.hasFixedBackendName) {
-      // Box fields and certain native fields must be given a specific name.
-      // Native names must not contain '$'. We rely on this to avoid clashes.
-      assert(element is BoxFieldElement ||
-          enclosingClass.isNative && !element.fixedBackendName.contains(r'$'));
+      // Certain native fields must be given a specific name. Native names must
+      // not contain '$'. We rely on this to avoid clashes.
+      assert(enclosingClass.isNative &&
+             !element.fixedBackendName.contains(r'$'));
 
       return element.fixedBackendName;
     }
 
+    // Instances of BoxFieldElement are special. They are already created with
+    // a unique and safe name. However, as boxes are not really instances of
+    // classes, the usual naming scheme that tries to avoid name clashes with
+    // super classes does not apply. We still do not mark the name as a
+    // fixedBackendName, as we want to allow other namers to do something more
+    // clever with them.
+    if (element is BoxFieldElement) {
+      return element.name;
+    }
+
     // If the name of the field might clash with another field,
     // use a mangled field name to avoid potential clashes.
     // Note that if the class extends a native class, that native class might
@@ -972,20 +982,6 @@ class Namer implements ClosureNamer {
     return name;
   }
 
-  /// Generate a unique name for the [id]th closure variable, with proposed name
-  /// [name].
-  ///
-  /// The result is used as the name of [BoxFieldElement]s and
-  /// [ClosureFieldElement]s, and must therefore be unique to avoid breaking an
-  /// invariant in the element model (classes cannot declare multiple fields
-  /// with the same name).
-  ///
-  /// Since the result is used as an element name, it will later show up as a
-  /// *proposed name* when the element is passed to [instanceFieldPropertyName].
-  String getClosureVariableName(String name, int id) {
-    return "${name}_$id";
-  }
-
   /**
    * Returns a proposed name for the given top-level or static element.
    * The returned id is guaranteed to be a valid JS-id.