Add ImportElement and ExportElement

pkg/compiler/lib/src/use_unused_api.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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
@@ skipping 126 matching lines @@
     ..asClassNode()
     ..asCombinator()
     ..asConditional()
     ..asContinueStatement()
     ..asEnum()
     ..asErrorExpression()
     ..asExport()
     ..asFor()
     ..asFunctionDeclaration()
     ..asIf()
+    ..asImport()
     ..asLabeledStatement()
     ..asLibraryDependency()
     ..asLibraryName()
     ..asLiteralDouble()
     ..asLiteralList()
     ..asLiteralMap()
     ..asLiteralMapEntry()
     ..asLiteralNull()
     ..asLiteralSymbol()
     ..asMetadata()
@@ skipping 181 matching lines @@
 
 class TreeVisitor1 extends tree_ir.ExpressionVisitor1
                       with tree_ir.StatementVisitor1 {
   noSuchMethod(inv) {}
 }
 
 useTreeVisitors() {
   new TreeVisitor1().visitExpression(null, null);
   new TreeVisitor1().visitStatement(null, null);
 }