Implement script tags.

dart/frog/leg/tree/nodes.dart

 // Copyright (c) 2011, 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.
 
 interface Visitor<R> {
   R visitBlock(Block node);
   R visitBreakStatement(BreakStatement node);
   R visitCatchBlock(CatchBlock node);
   R visitClassNode(ClassNode node);
   R visitConditional(Conditional node);
@@ skipping 1338 matching lines @@
   final LiteralString argument;
   final Identifier prefixIdentifier;
   final LiteralString prefix;
 
   final Token beginToken;
   final Token endToken;
 
   ScriptTag(this.tag, this.argument, this.prefixIdentifier, this.prefix,
             this.beginToken, this.endToken);
 
+  bool isImport() => tag.source == const SourceString("import");
+  bool isSource() => tag.source == const SourceString("source");
+  bool isLibrary() => tag.source == const SourceString("library");
+
   ScriptTag asScriptTag() => this;
 
   accept(Visitor visitor) => visitor.visitScriptTag(this);
 
   visitChildren(Visitor visitor) {
     tag.accept(visitor);
     argument.accept(visitor);
     if (prefixIdentifier !== null) prefixIdentifier.accept(visitor);
     if (prefix !== null) prefix.accept(visitor);
   }
@@ skipping 74 matching lines @@
 
   visitChildren(Visitor visitor) {
     formals.accept(visitor);
     block.accept(visitor);
   }
 
   Token getBeginToken() => catchKeyword;
 
   Token getEndToken() => block.getEndToken();
 }