Fix JS-backend when MirrorsUsed target a static field, but there is no mirrors usage.

sdk/lib/_internal/compiler/implementation/js_emitter/class_emitter.dart

 // Copyright (c) 2013, 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.
 
 part of dart2js.js_emitter;
 
 class ClassEmitter extends CodeEmitterHelper {
   /**
    * Documentation wanted -- johnniwinther
    *
@@ skipping 79 matching lines @@
   }
 
   /// Returns `true` if fields added.
   bool emitFields(Element element,
                   ClassBuilder builder,
                   String superName,
                   { bool classIsNative: false,
                     bool emitStatics: false,
                     bool onlyForRti: false }) {
     assert(!emitStatics || !onlyForRti);
-    bool isClass = false;
-    bool isLibrary = false;
-    if (element.isClass()) {
-      isClass = true;
-    } else if (element.isLibrary()) {
-      isLibrary = false;
+    if (element.isLibrary()) {
       assert(invariant(element, emitStatics));
-    } else {
+    } else if (!element.isClass()) {
       throw new SpannableAssertionFailure(
           element, 'Must be a ClassElement or a LibraryElement');
     }
     if (emitStatics) {
       assert(invariant(element, superName == null, message: superName));
     } else {
       assert(invariant(element, superName != null));
       String nativeName =
           namer.getPrimitiveInterceptorRuntimeName(element);
       if (nativeName != null) {
@@ skipping 482 matching lines @@
       computeTypeVariable =
           js(r'this.$builtinTypeInfo && this.$builtinTypeInfo[#]', index);
     }
     jsAst.Expression convertRtiToRuntimeType =
         namer.elementAccess(compiler.findHelper('convertRtiToRuntimeType'));
     builder.addProperty(
         name, js.fun(
             [], [js.return_(convertRtiToRuntimeType(computeTypeVariable))]));
   }
 }