Resolve some static warnings.

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 78966e3d53a6377f8f0ba3dbbae4f16b52a515d3..768a0726f57d7103af5c9b87bd80c87e884f9c3d 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -785,9 +785,10 @@ class Namer {
     // 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. So we can directly grab a name.
-    if (element is JSEntity) {
+    Entity asEntity = element;
+    if (asEntity is JSEntity) {
       return _disambiguateInternalMember(element,
-                                         () => element.declaredEntity.name);
+                                         () => asEntity.declaredEntity.name);
     }
 
     // If the name of the field might clash with another field,
@@ -1012,8 +1013,9 @@ class Namer {
     if (newName == null) {
       String name = proposeName();
 
-      if (element is PrivatelyNamedJSEntity) {
-        NamingScope scope = _getPrivateScopeFor(element);
+      Entity asEntity = element;
+      if (asEntity is PrivatelyNamedJSEntity) {
+        NamingScope scope = _getPrivateScopeFor(asEntity);
         newName = getFreshName(scope, name,
                                sanitizeForAnnotations: true,
                                sanitizeForNatives: false);