Change MirrorSystem.libraries to Map<Uri, String>

sdk/lib/_internal/compiler/implementation/filenames.dart

Index: sdk/lib/_internal/compiler/implementation/filenames.dart
diff --git a/sdk/lib/_internal/compiler/implementation/filenames.dart b/sdk/lib/_internal/compiler/implementation/filenames.dart
index 198a28f38822e46cb25223cb04c6ab104cf00ac6..46bc421a1cc75abe7d902f19b155915ef3bc0765 100644
--- a/sdk/lib/_internal/compiler/implementation/filenames.dart
+++ b/sdk/lib/_internal/compiler/implementation/filenames.dart
@@ -21,9 +21,8 @@ String uriPathToNative(String path) {
   return new Path(path).toNativePath();
 }
 
-Uri getCurrentDirectory() {
-  final String dir = nativeToUriPath(new File('.').fullPathSync());
-  return new Uri.fromComponents(scheme: 'file', path: appendSlash(dir));
-}
+final Uri currentDirectory = new Uri.fromComponents(
+    scheme: 'file',
+    path: appendSlash(nativeToUriPath(new File('.').fullPathSync())));
 
 String appendSlash(String path) => path.endsWith('/') ? path : '$path/';