Add support for the package: directive in Frog and dart2js.

frog/library.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 LibraryImport {
   final String prefix;
   final Library library;
   final SourceSpan span;
   LibraryImport(this.library, [this.prefix, this.span]);
 }
@@ skipping 41 matching lines @@
   bool get isCore() => this == world.corelib;
   bool get isCoreImpl() => this == world.coreimpl;
 
   // TODO(jmesserly): we shouldn't be special casing DOM anywhere.
   bool get isDom() => this == world.dom;
 
   SourceSpan get span() => new SourceSpan(baseSource, 0, 0);
 
   String makeFullPath(String filename) {
     if (filename.startsWith('dart:')) return filename;
+    if (filename.startsWith('package:')) return filename;
     // TODO(jmesserly): replace with node.js path.resolve
     if (filename.startsWith('/')) return filename;
     if (filename.startsWith('file:///')) return filename;
     if (filename.startsWith('http://')) return filename;
     if (const RegExp('^[a-zA-Z]:/').hasMatch(filename)) return filename;
     return joinPaths(sourceDir, filename);
   }
 
   /** Adds an import to the library. */
   addImport(String fullname, String prefix, SourceSpan span) {
@@ skipping 431 matching lines @@
 
   void visitFunctionDefinition(FunctionDefinition node) {
     currentType.addMethod(node.name.name, node);
   }
 
   void visitFunctionTypeDefinition(FunctionTypeDefinition node) {
     var type = library.addType(node.func.name.name, node, false);
     type.addMethod(':call', node.func);
   }
 }