Add new core ("dart:") libraries to frog so that apidoc can find them.

frog/leg/lib/io.dart

Index: frog/leg/lib/io.dart
diff --git a/frog/leg/lib/io.dart b/frog/leg/lib/io.dart
index 6ee3404e0bc2d456f358be38c772627a7fad401a..ea3c69b2d130b650c114d580434b07a9f0aac700 100644
--- a/frog/leg/lib/io.dart
+++ b/frog/leg/lib/io.dart
@@ -16,6 +16,7 @@
 // #import("dart:nativewrappers");
 #import("dart:uri");
 #source('../../../runtime/bin/buffer_list.dart');
+#source('../../../runtime/bin/common.dart');
 #source('../../../runtime/bin/chunked_stream.dart');
 #source('../../../runtime/bin/directory.dart');
 // Uses native keyword.
@@ -78,6 +79,13 @@ class _Process implements Process {
   }
 }
 
+class _ServerSocket implements ServerSocket {
+  factory _ServerSocket(String bindAddress, int port, int backlog) {
+    throw new UnsupportedOperationException(
+        'new ServerSocket($bindAddress, $port, $backlog)');
+  }
+}
+
 class _Socket implements Socket {
   factory Socket(String host, int port) {
     throw new UnsupportedOperationException('new Socket($host, $int)');