Fix dart2js mock of dart:io, to incorporate new File.fromPath and Directory.fromPath.

lib/compiler/implementation/lib/io.dart

Index: lib/compiler/implementation/lib/io.dart
diff --git a/lib/compiler/implementation/lib/io.dart b/lib/compiler/implementation/lib/io.dart
index 9191d3f11473b30720830a5e0b1499d30863b445..751a5052d4eec47c166a39d053aa27101f8a77aa 100644
--- a/lib/compiler/implementation/lib/io.dart
+++ b/lib/compiler/implementation/lib/io.dart
@@ -62,6 +62,10 @@ class _File {
   factory File(arg) {
     throw new UnsupportedOperationException('new File($arg)');
   }
+
+  factory File.fromPath(arg) {
+    throw new UnsupportedOperationException('new File.fromPath($arg)');
+  }
 }
 
 class _Platform {
@@ -91,6 +95,10 @@ class _Directory {
     throw new UnsupportedOperationException('new Directory($arg)');
   }
 
+  factory Directory.fromPath(arg) {
+    throw new UnsupportedOperationException('new Directory.fromPath($arg)');
+  }
+
   factory Directory.current() {
     throw new UnsupportedOperationException('new Directory.current()');
   }