Adding 7-zip to third party so that we can zip SDKs on Windows.

dart/third_party/7zip/license.txt

+      7-Zip Command line version
+      ~~~~~~~~~~~~~~~~~~~~~~~~~~
+      License for use and distribution
+      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+      7-Zip Copyright (C) 1999-2010 Igor Pavlov.
+
+      7za.exe is distributed under the GNU LGPL license
+
+      Notes: 
+        You can use 7-Zip on any computer, including a computer in a commercial 
+        organization. You don't need to register or pay for 7-Zip.
+
+
+      GNU LGPL information
+      --------------------
+
+        This library is free software; you can redistribute it and/or
+        modify it under the terms of the GNU Lesser General Public
+        License as published by the Free Software Foundation; either
+        version 2.1 of the License, or (at your option) any later version.
+
+        This library is distributed in the hope that it will be useful,
+        but WITHOUT ANY WARRANTY; without even the implied warranty of
+        MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+        Lesser General Public License for more details.
+
+        You can receive a copy of the GNU Lesser General Public License from 
+        http://www.gnu.org/