esbuild `import.meta` warning

Bug Report Quick Checklist

• I am on the latest version of Snowpack & all plugins.
• I use package manager npm
• I run Snowpack on macOS
• I run Snowpack on Node.js 12.20.1

Describe the bug

The HMR import.meta check causes this esbuild warning when targetting browsers that don’t support import.meta:

[snowpack] ! optimizing build...
 > dist/index.js:2:0: warning: "import.meta" is not available in the configured target environment and will be empty
    2 │ import.meta.env = __SNOWPACK_ENV__;
      ╵ ~~~~~~~~~~~

1 warning
[
  {
    detail: undefined,
    location: {
      column: 0,
      file: 'dist/index.js',
      length: 11,
      line: 2,
      lineText: 'import.meta.env = __SNOWPACK_ENV__;',
      namespace: ''
    },
    notes: [],
    text: '"import.meta" is not available in the configured target environment and will be empty'
  }
]

To Reproduce

1. npx create-snowpack-app my-app --template @snowpack/app-template-svelte
2. cd my-app
3. Set optimize.target to 'es2019' in snowpack.config.js
4. npm run build

Expected behavior

Ideally it should be possible to use HMR while building for browsers that don’t support import.meta without getting warnings