[vm/aot] Drop Library::dependencies_ early.

Even though this field is never written into snapshot dropping it
eagerly allows to compact away a lot of entries in the symbols
table, which are otherwise retained.

On the large internal application this gives the following improvement:

Breakdown by object type:
+--------------------------+--------------+---------+
| Type                     | Size (Bytes) | Percent |
+--------------------------+--------------+---------+
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ArtificialRoot           |           -7 |   0.01% |
| OneByteString            |        -1603 |   1.26% |
| Array                    |        -3206 |   2.51% |
| (RO) _OneByteString      |      -122688 |  96.22% |
+--------------------------+--------------+---------+

Total: -127504 bytes
       -0.39% of old size
Change-Id: Ib6993e3036ff6b36515a17ee7f60175e1188c8fe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153768
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>

Author: mraleph

runtime/vm/object.cc

@@ -12135,6 +12135,7 @@ void Library::DropDependenciesAndCaches() const {
   StorePointer(&raw_ptr()->resolved_names_, Array::null());
   StorePointer(&raw_ptr()->exported_names_, Array::null());
   StorePointer(&raw_ptr()->loaded_scripts_, Array::null());
+  StorePointer(&raw_ptr()->dependencies_, Array::null());
 }
 
 void Library::AddImport(const Namespace& ns) const {