Update and enable embind tests to run with the Closure Compiler

This change adds a pass to the embind tests to run them with the Closure
Compiler enabled.

Because embind.test.js will be passed through the Closure Compiler, name-
mangling will occur on unquoted references to embound classes and functions.
However, because embound symbols are exposed via string literals, this results
in a mismatch.

The tests have now been updated so that all references to embound symbols are
referred to by string literals (i.e. object["property"] syntax) instead of just
by name (i.e. object.property syntax).

A set of Closure externs for Underscore were also added, because Underscore
(which is used by the embind test adapter) exposes some symbols by string
literal as well.