Add --sky support in mojo_shell.py.

mojo/devtools/common/devtoolslib/http_server.py

Index: mojo/devtools/common/devtoolslib/http_server.py
diff --git a/mojo/devtools/common/devtoolslib/http_server.py b/mojo/devtools/common/devtoolslib/http_server.py
index 6db07173a1db1215d3cc214faf3915fbff77ce03..0038392536f4f73d85f90e8c4e97d463781adbeb 100644
--- a/mojo/devtools/common/devtoolslib/http_server.py
+++ b/mojo/devtools/common/devtoolslib/http_server.py
@@ -140,6 +140,13 @@ def _GetHandlerClassForPath(mappings):
       # prefix at the end of |mappings|.
       assert False
 
+    def guess_type(self, path):
+      if path.endswith('.dart'):
+        return 'application/dart'
+      elif path.endswith('.sky'):
+        return 'text/sky'
+      return SimpleHTTPServer.SimpleHTTPRequestHandler.guess_type(self, path)
+
     def log_message(self, *_):
       """Override the base class method to disable logging."""
       pass