Don't delete generated files from xmodule-assets

xmodule-assets creates coffeescript files in the output directories. On
its next run, it used to delete the javascript files compiled from those
coffee files. Now it doesn't which should make coffee have to do less
work.

Fixes LMS-451

CHANGELOG.rst

@@ -5,6 +5,10 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+XModule: Don't delete generated xmodule asset files when compiling (for
+instance, when XModule provides a coffeescript file, don't delete
+the associated javascript)
+
 Common: Make rake provide better error messages if packages are missing.
 
 Common: Repairs development documentation generation by sphinx.

common/lib/xmodule/xmodule/static_content.py

@@ -121,15 +121,23 @@ def _write_js(output_root, classes):
             type=filetype)
         contents[filename] = fragment
 
-    _write_files(output_root, contents)
+    _write_files(output_root, contents, {'.coffee': '.js'})
 
     return [output_root / filename for filename in contents.keys()]
 
 
-def _write_files(output_root, contents):
+def _write_files(output_root, contents, generated_suffix_map=None):
     _ensure_dir(output_root)
-    for extra_file in set(output_root.files()) - set(contents.keys()):
-        extra_file.remove_p()
+    to_delete = set(file.basename() for file in output_root.files()) - set(contents.keys())
+
+    if generated_suffix_map:
+        for output_file in contents.keys():
+            for suffix, generated_suffix in generated_suffix_map.items():
+                if output_file.endswith(suffix):
+                    to_delete.discard(output_file.replace(suffix, generated_suffix))
+
+    for extra_file in to_delete:
+        (output_root / extra_file).remove_p()
 
     for filename, file_content in contents.iteritems():
         (output_root / filename).write_bytes(file_content)