Add minified names to sourcemaps

This change:

* introduces a sourcemap extension to store minified names from the global and
instance namespaces.

* adds a test suite to test that this information is accurate

This is just an initial step: at this time names come from proposed names, which
include extra stuff we don't want. On a later CL I plan to change this to be
able to get the original name instead.

For example, I want to add a test for translating an instance method, right now
we would get "@method@0@" or "method$0" instead of "method".

Change-Id: I411c3bea96446e29e80581750250349799332b9a
Reviewed-on: https://dart-review.googlesource.com/65660
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>