Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/leg/lib/io.dart

Index: dart/frog/leg/lib/io.dart
diff --git a/dart/frog/leg/lib/io.dart b/dart/frog/leg/lib/io.dart
index ea3c69b2d130b650c114d580434b07a9f0aac700..40484505ed1a7c89e382a7e17fb2170668955170 100644
--- a/dart/frog/leg/lib/io.dart
+++ b/dart/frog/leg/lib/io.dart
@@ -68,6 +68,10 @@ class _Directory implements Directory {
   factory Directory(arg) {
     throw new UnsupportedOperationException('new Directory($arg)');
   }
+
+  factory Directory.current() {
+    throw new UnsupportedOperationException('new Directory.current()');
+  }
 }
 
 class _Process implements Process {
@@ -88,7 +92,7 @@ class _ServerSocket implements ServerSocket {
 
 class _Socket implements Socket {
   factory Socket(String host, int port) {
-    throw new UnsupportedOperationException('new Socket($host, $int)');
+    throw new UnsupportedOperationException('new Socket($host, $port)');
   }
 }