Remove a few warnings.

lib/compiler/implementation/dart_backend/placeholder_collector.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.
 
 class LocalPlaceholder implements Hashable {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
   int hashCode() => identifier.hashCode();
   String toString() =>
@@ skipping 421 matching lines @@
         if (node.name !== null) {
           assert(node.name is Identifier);
           tryMakeLocalPlaceholder(element, node.name);
         }
       }
     }
     node.visitChildren(this);
   }
 
   visitClassNode(ClassNode node) {
-    assert(currentElement is ClassElement);
-    makeElementPlaceholder(node.name, currentElement);
+    ClassElement classElement = currentElement;
+    makeElementPlaceholder(node.name, classElement);
     node.visitChildren(this);
     if (node.typeParameters !== null) {
       // Another poor man resolution.
       final typeVariableTypes =
-          new List<Type>.from(currentElement.typeVariables);
+          new List<Type>.from(classElement.typeVariables);
       int i = 0;
       for (TypeVariable typeVariable in node.typeParameters) {
         makeTypePlaceholder(typeVariable.name, typeVariableTypes[i]);
         i++;
       }
     }
     if (node.defaultClause !== null) {
       // Can't just visit class node's default clause because of the bug in the
       // resolver, it just crashes when it meets type variable.
-      Type defaultType = (currentElement as ClassElement).defaultClass;
+      Type defaultType = classElement.defaultClass;
       assert(defaultType !== null);
       makeTypePlaceholder(node.defaultClause.typeName, defaultType);
       visit(node.defaultClause.typeArguments);
     }
   }
 
   visitTypedef(Typedef node) {
     assert(currentElement is TypedefElement);
     makeElementPlaceholder(node.name, currentElement);
     node.visitChildren(this);
   }
 }