Add PluginWatcher to watch for new plugins that need to be run

pkg/analysis_server/lib/src/plugin/plugin_locator.dart

Index: pkg/analysis_server/lib/src/plugin/plugin_locator.dart
diff --git a/pkg/analysis_server/lib/src/plugin/plugin_locator.dart b/pkg/analysis_server/lib/src/plugin/plugin_locator.dart
index ac7c1f0c399cdfe4547cbc8cb63d1bc348b77fca..968ee5384011d96ed66c33dae242ae9a7b6f4b88 100644
--- a/pkg/analysis_server/lib/src/plugin/plugin_locator.dart
+++ b/pkg/analysis_server/lib/src/plugin/plugin_locator.dart
@@ -65,10 +65,12 @@ class PluginLocator {
         YamlNode contents = document.contents;
         if (contents is YamlMap) {
           String pluginPath = contents[analysisPluginKey];
-          Folder pluginFolder =
-              packageFolder.getChildAssumingFolder(pluginPath);
-          if (pluginFolder.exists) {
-            return pluginFolder.path;
+          if (pluginPath != null) {
+            Folder pluginFolder =
+                packageFolder.getChildAssumingFolder(pluginPath);
+            if (pluginFolder.exists) {
+              return pluginFolder.path;
+            }
           }
         }
       } catch (exception) {