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

frog/reader.dart

Index: frog/reader.dart
diff --git a/frog/reader.dart b/frog/reader.dart
index 222fca254e6ecd58b645dc082dd348d2a0a4c699..66fe269bf397e4a686be7c24f8fcf805398f6fad 100644
--- a/frog/reader.dart
+++ b/frog/reader.dart
@@ -43,10 +43,17 @@ class LibraryReader {
     }
   }
 
-  SourceFile readFile(String fullname) {
-    var filename = _specialLibs[fullname];
+  SourceFile readFile(String fullName) {
+    String filename;
+    if (fullName.startsWith('package:')) {
+      filename = joinPaths(dirname(options.dartScript),
+          joinPaths('packages', fullName.substring('package:'.length)));
+    } else {
+      filename = _specialLibs[fullName];
+    }
+
     if (filename == null) {
-      filename = fullname;
+      filename = fullName;
     }
 
     if (world.files.fileExists(filename)) {