Fix warnings in dart2js.

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

Index: sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
index 4168c72fa3789a2d3b5b8ff48505b263ff1cc93c..836d99914405130d3f4ed6969d482983c22f1bcd 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
@@ -595,14 +595,17 @@ class ReferencedElementCollector extends Visitor {
   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;
-
-  visitNode(Node node) { node.visitChildren(this); }
+  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);
+  }
 
   visitTypeAnnotation(TypeAnnotation typeAnnotation) {
     // We call [resolveReturnType] to allow having 'void'.