chore: guard against property renaming for Angular-Polymer interop types (#5144)
chore: guard against property renaming for Angular-Polymer interop types (#5144)

The JS Closure compiler offers stricter optimizations such as property renaming.
In our app, the Polymer bundle and Angular bundle communicate using a set of
interop types defined in TypeScript.  To ensure that Angular code referencing
fields do not fail due to the Closure renaming in a separately-built Polymer
bundle, we use the `declare` keyword, respected internally as a way to define
interfaces as externs.

While TensorBoard does not currently enable the property renaming optimization,
this moves it one step closer to doing so.

Googlers, see test sync cl/385887792, property renaming debugging cl/385883631
Related: b/193918192
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard