Fix warnings in dart2js.

sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart

 // Copyright (c) 2012, 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 dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 577 matching lines @@
  * used in signatures, as/is operators or in super clauses
  * (just to name a few).  Retraverse AST to pick those up.
  */
 class ReferencedElementCollector extends Visitor {
   final Compiler compiler;
   final Element rootElement;
   final TreeElements treeElements;
   final newTypedefElementCallback;
   final newClassElementCallback;
 
-  ReferencedElementCollector(
-      this.compiler,
-      Element rootElement, this.treeElements,
-      this.newTypedefElementCallback, this.newClassElementCallback)
-      : this.rootElement = (rootElement is VariableElement)
-          ? (rootElement as VariableElement).variables : rootElement;
+  ReferencedElementCollector(this.compiler,
+                             Element rootElement,
+                             this.treeElements,
+                             this.newTypedefElementCallback,
+                             this.newClassElementCallback)
+      : this.rootElement =
+          rootElement is VariableElement ? rootElement.variables : rootElement;
 
-  visitNode(Node node) { node.visitChildren(this); }
+  visitNode(Node node) {
+    node.visitChildren(this);
+  }
 
   visitTypeAnnotation(TypeAnnotation typeAnnotation) {
     // We call [resolveReturnType] to allow having 'void'.
     final type = compiler.resolveReturnType(rootElement, typeAnnotation);
     Element typeElement = type.element;
     if (typeElement.isTypedef()) newTypedefElementCallback(typeElement);
     if (typeElement.isClass()) newClassElementCallback(typeElement);
     typeAnnotation.visitChildren(this);
   }
 
@@ skipping 13 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().canonicalUri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Iterable<Element> elements) =>
     sorted(elements, compareElements);