Fixes #410

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index a411957c9e51b597054c81dfdf3a415c5cd7dc35..dc64c5bb8d44c6ac9f0f03f52a83d044e09f708f 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -3423,8 +3423,12 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ClosureAnnotator {
     }
 
     // Dart "extension" methods. Used for JS Array, Boolean, Number, String.
+    var baseType = type;
+    while (baseType is TypeParameterType) {
+      baseType = baseType.element.bound;
+    }
     if (allowExtensions &&
-        _extensionTypes.contains(type.element) &&
+        _extensionTypes.contains(baseType.element) &&
         !_isObjectProperty(name)) {
       return js.call('dartx.#', _propertyName(name));
     }