Add ImportElement and ExportElement

pkg/compiler/lib/src/tree/unparser.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 tree;
 
 String unparse(Node node, {minify: true}) {
   Unparser unparser = new Unparser(minify: minify);
   unparser.unparse(node);
   return unparser.result;
@@ skipping 680 matching lines @@
     newline();
     indentMore();
     visit(node.labelsAndCases);
     if (node.isDefaultCase) {
       write('default:');
     }
     unparseBlockStatements(node.statements);
     indentLess();
   }
 
+  unparseLibraryName(String libraryName) {
+    write('library $libraryName;');
+    newline();
+  }
+
   unparseImportTag(String uri, {String prefix,
                                 List<String> shows: const <String>[],
                                 bool isDeferred: false}) {
     String deferredString = isDeferred ? ' deferred' : '';
     String prefixString = prefix == null ? '' : ' as $prefix';
     String showString = shows.isEmpty ? '' : ' show ${shows.join(", ")}';
     write('import "$uri"$deferredString$prefixString$showString;');
     newline();
   }
 
@@ skipping 139 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }