add ImportMeta to closure_externs.js
add ImportMeta to closure_externs.js

ImportMeta is declared in [lib.es5.ts](https://github.com/microsoft/TypeScript/blob/610fa28de55d37b0e46fa1cf6394da117d29be9d/lib/lib.es5.d.ts#L614)
and is referened in
[@types/node](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/2833781317b9e009c81195998b1f67c9e67d1c33/types/node/globals.d.ts#L24).
Other than `ImportMeta`, generating externs from `@types/node` at least
works.

angular/tsickle,angular/tsickle