fix babel proposal-decorators use legacy the query decorator return undefined

lit / lit-element

LEGACY REPO. This repository is for maintenance of the legacy LitElement library. The LitElement base class is now part of the Lit library, which is developed in the lit monorepo.

BSD 3-Clause "New" or "Revised" License

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Fixes #1121

I Write lit-element like this:

@customElement('custom-div')
export class CustomDiv extends LitElement {
  @query('#btn') btn: HTMLButtonElement

  // ....
}

And I also used babel(v7) and webpack(v4), I set @babel/plugin-proposal-decorators legacy was true

babel.config.json:

{
  "presets": [
    "@babel/env",
    "@babel/preset-typescript"
  ],
  "plugins": [
    [
      "@babel/plugin-proposal-decorators", {
        "legacy": true
      }
    ],
    [
      "@babel/plugin-proposal-class-properties"
    ]
  ]
}

When I builded i got an error: this.btn is undefined. If you change legacy todecoratorsBeforeExport, this issue won't be show.

I founded the issue cased by legacyQuery function didn't use 'return' Statement.

lit / lit-element

fix babel proposal-decorators use legacy the query decorator return undefined #1138