Use entities in CommonElements interface.

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

Index: pkg/compiler/lib/src/js_backend/backend_impact.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_impact.dart b/pkg/compiler/lib/src/js_backend/backend_impact.dart
index f2df00dd4a7ac4ff0ca8280bb87733c254d35e9f..97da9b7e0fd4dfb095388bb09ad194a5f086e8cd 100644
--- a/pkg/compiler/lib/src/js_backend/backend_impact.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_impact.dart
@@ -176,7 +176,7 @@ class BackendImpacts {
                 helpers.unresolvedTopLevelMethodError,
                 helpers.unresolvedTopLevelGetterError,
                 helpers.unresolvedTopLevelSetterError,
-                commonElements.symbolConstructor.declaration,
+                commonElements.symbolConstructor,
               ]
             : [
                 helpers.throwNoSuchMethod,
@@ -295,7 +295,7 @@ class BackendImpacts {
   BackendImpact get constSymbol {
     return _constSymbol ??= new BackendImpact(
         instantiatedClasses: [commonElements.symbolClass],
-        staticUses: [commonElements.symbolConstructor.declaration]);
+        staticUses: [commonElements.symbolConstructor]);
   }
 
   /// Helper for registering that `int` is needed.