Support TS isolatedModules

When I try to yarn build my Nextjs project using typescript and vega-lite, I get this error:

The following mandatory changes were made to your tsconfig.json:

	- isolatedModules was set to true (requirement for babel)

Failed to compile.

./node_modules/vega-lite/src/compositemark/index.ts:28:9
Type error: Re-exporting a type when the '--isolatedModules' flag is provided requires using 'export type'.

  26 | } from './errorbar';
  27 |
> 28 | export {BoxPlotConfig} from './boxplot';
     |         ^
  29 | export {ErrorBandConfigMixins} from './errorband';
  30 | export {ErrorBarConfigMixins} from './errorbar';
  31 |
error Command failed with exit code 1.

I think this is pointing to this line: https://github.com/vega/vega-lite/blob/22057b18d54e71be4cc7c959c5f83d43d765a697/src/compositemark/index.ts#L28 Do we need to update these export statements to export type?

I doubt that the vega-lite codebase is not compatible with Babel, since vega-lite is currently producing ES2015 output. So maybe my TypeScript configuration is wrong?

Here is my tsconfig.json:

{
  "compilerOptions": {
    "target": "es6",
    "lib": [
      "dom",
      "dom.iterable",
      "esnext"
    ],
    "allowJs": true,
    "skipLibCheck": true,
    "strict": false,
    "forceConsistentCasingInFileNames": true,
    "noEmit": true,
    "esModuleInterop": true,
    "module": "esnext",
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "jsx": "preserve",
    "baseUrl": ".",
    "isolatedModules": true,
    "downlevelIteration": true
  },
  "include": [
    "next-env.d.ts",
    "**/*.ts",
    "**/*.tsx"
  ],
  "exclude": [
    "node_modules"
  ]
}

[sid-kap]

Sure! I can do that

[domoritz]

We don’t support isolatedModules right now. Do you want to help us support it? What you need to do is set "isolatedModules": true in our tsconfig, run yarn tsc and fix any errors that come up.