Unify most of our utf8 implementations. This takes the implementation

frog/file_system_vm.dart

Index: frog/file_system_vm.dart
===================================================================
--- frog/file_system_vm.dart	(revision 5167)
+++ frog/file_system_vm.dart	(working copy)
@@ -5,7 +5,7 @@
 #library('file_system_vm');
 #import('dart:io');
 #import('file_system.dart');
-#import('dart:utf8');
+#import('dart:utf');
 
 /** File system implementation using the vm api's. */
 class VMFileSystem implements FileSystem {
@@ -21,7 +21,7 @@
     var buffer = new List<int>(length);
     var bytes = file.readListSync(buffer, 0, length);
     file.closeSync();
-    return new String.fromCharCodes(Utf8Decoder.decodeUtf8(buffer));
+    return new String.fromCharCodes(new Utf8Decoder(buffer).decodeRest());
   }
 
   bool fileExists(String filename) {