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.

src/index.ts

@@ -15,6 +15,12 @@
  * =============================================================================
  */
 
+// backend_cpu.ts and backend_webgl.ts are standalone files and should be
+// explicily included here. Below, there is an export from backend_webgl, but
+// that doesn't count since it's exporting a Typescript interface.
+import './kernels/backend_webgl';
+import './kernels/backend_cpu';
+
 import {BrowserUtil} from './browser_util';
 import * as environment from './environment';
 import {Environment} from './environment';