Describe the bug
Typing ==( generated overeager completion

To Reproduce
See attached movie file
https://user-images.githubusercontent.com/2319867/110220049-f1ed4980-7e77-11eb-9804-10415d1bc655.mov

Expected behavior
No insertion of autocomplete results

Versions (please complete the following information):

• VS Code version: 1.54.1
• Dart extension version: 3.20.1
• Dart/Flutter SDK version: 2.0
• LSP preview enabled