Fixes #410

lib/src/codegen/js_codegen.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 3405 matching lines @@
       name = 'set';
     } else if (name == '-' && unary) {
       name = 'unary-';
     } else if (name == 'constructor' || name == 'prototype') {
       // This uses an illegal (in Dart) character for a member, avoiding the
       // conflict. We could use practically any character for this.
       name = '+$name';
     }
 
     // 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));
     }
 
     return _propertyName(name);
   }
 
   bool _externalOrNative(node) =>
       node.externalKeyword != null || _functionBody(node) is NativeFunctionBody;
 
@@ skipping 178 matching lines @@
 
 /// A special kind of element created by the compiler, signifying a temporary
 /// variable. These objects use instance equality, and should be shared
 /// everywhere in the tree where they are treated as the same variable.
 class TemporaryVariableElement extends LocalVariableElementImpl {
   TemporaryVariableElement.forNode(Identifier name) : super.forNode(name);
 
   int get hashCode => identityHashCode(this);
   bool operator ==(Object other) => identical(this, other);
 }