Skip npm step if node_modules already exists (#640)

See https://github.com/WebAssembly/waterfall/pull/670

emsdk.py

@@ -1848,8 +1848,12 @@ class Tool(object):
     if getattr(self, 'custom_install_script', None) == 'emscripten_npm_install':
       # upstream tools have hardcoded paths that are not stored in emsdk_manifest.json registry
       install_path = 'upstream' if 'releases-upstream' in self.version else 'fastcomp'
-      if not emscripten_npm_install(self, os.path.join(emsdk_path(), install_path, 'emscripten')):
-        exit_with_error('post-install step failed: emscripten_npm_install')
+      emscripten_dir = os.path.join(emsdk_path(), install_path, 'emscripten')
+      # Older versions of the sdk did not include the node_modules directory
+      # and require `npm ci` to be run post-install
+      if not os.path.exists(os.path.join(emscripten_dir, 'node_modules')):
+        if not emscripten_npm_install(self, emscripten_dir):
+          exit_with_error('post-install step failed: emscripten_npm_install')
 
     print("Done installing SDK '" + str(self) + "'.")
     return True

test/test.py

@@ -205,6 +205,10 @@ int main() {
     # TODO; test on latest as well
     check_call(upstream_emcc + ' hello_world.c')
 
+  def test_closure(self):
+    # Specificlly test with `--closure` so we know that node_modules is working
+    check_call(upstream_emcc + ' hello_world.c --closure=1')
+
   def test_specific_old(self):
     print('test specific release (old, using sdk-* notation)')
     run_emsdk('install sdk-fastcomp-1.38.31-64bit')