Unparse new library syntax.

dart/lib/compiler/implementation/scanner/scanner_task.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 ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name => 'Scanner';
 
   void scanLibrary(LibraryElement library) {
     var compilationUnit = library.entryCompilationUnit;
@@ skipping 40 matching lines @@
           compiler.cancel("duplicated library declaration", node: tag);
         } else {
           library.libraryTag = tag;
         }
       } else if (tag.isPart) {
         StringNode uri = tag.uri;
         Uri resolved = base.resolve(uri.dartString.slowToString());
         tagState = checkTag(TagState.SOURCE, tag);
         loadPart(tag, resolved, library);
       } else {
-        compiler.cancel("illegal script tag: ${tag.tag}", node: tag);
+        compiler.internalError("Unhandled library tag.", node: tag);
       }
     }
 
     // Apply patch, if any.
     if (library.uri.scheme == 'dart') {
       compiler.patchDartLibrary(library, library.uri.path);
     }
 
     // Now that we have processed all the source tags, it is safe to
     // start loading other libraries.
@@ skipping 137 matching lines @@
   static const int RESOURCE = 4;
 
   /** Next state. */
   static const List<int> NEXT =
       const <int>[NO_TAG_SEEN,
                   IMPORT, // Only one library tag is allowed.
                   IMPORT,
                   SOURCE,
                   RESOURCE];
 }