Remove some warnings and dead code in dart2js.

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 {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
   int hashCode() => identifier.hashCode();
   String toString() =>
@@ skipping 457 matching lines @@
       // coreLibPrefix.topLevels.
       Element typeElement = type.element;
       Element dynamicTypeElement = compiler.types.dynamicType.element;
       if (hasPrefix &&
           (typeElement.getLibrary() === coreLibrary ||
           typeElement === dynamicTypeElement)) {
         makeNullPlaceholder(node.typeName.asSend().receiver);
       } else {
         if (hasPrefix) {
           assert(node.typeName is Send);
-          assert(node.typeName.receiver is Identifier);
-          assert(node.typeName.selector is Identifier);
-          makeNullPlaceholder(node.typeName.receiver);
+          Send typeName = node.typeName;
+          assert(typeName.receiver is Identifier);
+          assert(typeName.selector is Identifier);
+          makeNullPlaceholder(typeName.receiver);
         }
         if (typeElement !== dynamicTypeElement) {
           makeTypePlaceholder(target, type);
         } else {
           if (!isDynamicType(node)) makeUnresolvedPlaceholder(target);
         }
       }
     }
     // Trying to differentiate new A.foo() and lib.A cases. In the latter case
     // we don't want to go deeper into typeName.
@@ skipping 139 matching lines @@
 
   visitBlock(Block node) {
     for (Node statement in node.statements.nodes) {
       if (statement is VariableDefinitions) {
         makeVarDeclarationTypePlaceholder(statement);
       }
     }
     node.visitChildren(this);
   }
 }