diff --git a/lib/legacy/class.html b/lib/legacy/class.html
index b828ecb047..7411d4bb15 100644
--- a/lib/legacy/class.html
+++ b/lib/legacy/class.html
@@ -312,7 +312,7 @@
      */
     Polymer.Class = function(info) {
       if (!info) {
-        Polymer._warn('Polymer.Class requires `info` argument');
+        console.warn('Polymer.Class requires `info` argument');
       }
       let klass = GenerateClassFromInfo(info, info.behaviors ?
         // note: mixinBehaviors ensures `LegacyElementMixin`.
diff --git a/test/unit/mixin-behaviors.html b/test/unit/mixin-behaviors.html
index 139b83b870..3a598b6318 100644
--- a/test/unit/mixin-behaviors.html
+++ b/test/unit/mixin-behaviors.html
@@ -495,11 +495,7 @@
   });
 
   test('behavior is null generates warning', function() {
-    var warned = false, oldWarn = Polymer._warn;
-    Polymer._warn = function(message) {
-      assert.match(message, /behavior is null/);
-      warned = true;
-    };
+    sinon.spy(console, 'warn');
     Polymer({
       is: 'behavior-null',
       behaviors: [
@@ -507,8 +503,8 @@
       ]
     });
     document.createElement('behavior-null');
-    assert.equal(warned, true, 'Null behaviour should generate warning');
-    Polymer.Base._warn = oldWarn;
+    assert.equal(console.warn.callCount, 1, 'Null behaviour should generate warning');
+    console.warn.restore();
   });
 
   test('behavior array is unique', function() {