Fix for no-op assist proposals.

pkg/analysis_server/lib/src/services/correction/assist_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/assist_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
index 2cd0ce6337a9ed4f14fd4109588703464c4afe88..5280a44f99de21cc2e27f6101751ee38c45492b9 100644
--- a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
@@ -125,6 +125,12 @@ class AssistProcessor {
     if (assistFile == null) {
       assistFile = file;
     }
+    // check is there are any edits
+    if (edits.isEmpty) {
+      _coverageMarker();
+      return;
+    }
+    // prepare file edit
     SourceFileEdit fileEdit = new SourceFileEdit(file);
     fileEdit.addAll(edits);
     // prepare Change