Fix build issue where module-import code gets dropped from the bundle (#987)
Fix build issue where module-import code gets dropped from the bundle (#987)

`backend_cpu.ts` and `backend_webgl.ts` are never included from `index.ts` even though there is
`export {WebGLTimingInfo} from './kernels/backend_webgl';`. That export is just a Typescript interface, and gets compiled away in the es5 code.

tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs