Use default develop script

package.json

@@ -16,7 +16,7 @@
     "clean:dev": "cd dev_mode && jlpm run clean",
     "clean:examples": "node buildutils/lib/clean-packages.js examples",
     "clean:packages": "node buildutils/lib/clean-packages.js packages",
-    "clean:slate": "python clean.py && pip install --global-option='--skip-npm' -v -e . && jlpm",
+    "clean:slate": "python clean.py && pip install -v -e . && jlpm",
     "clean:src": "jlpm run clean",
     "clean:test": "lerna run clean --scope \"@jupyterlab/test-*\"",
     "clean:utils": "cd buildutils && jlpm run clean",

setup.py

@@ -16,6 +16,7 @@ from setupbase import (
 )
 
 from setuptools import setup
+from setuptools.command.develop import develop
 
 
 NAME = 'jupyterlab'
@@ -84,6 +85,8 @@ cmdclass['jsdeps'] = combine_commands(
                 build_dir=pjoin(HERE, NAME, 'static'), npm=npm),
     command_for_func(check_assets)
 )
+# Use default develop - we can ensure core mode later if needed.
+cmdclass['develop'] = develop
 
 
 setup_args = dict(