Precompilation: drop unused types and type arguments and empty classes and libraries.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index c85afa2c21058d56586eadf69c79333f1ccf39b7..30c3c5de5e8fd86653720f50dc5dc75eaff695ce 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -3486,6 +3486,11 @@ void Class::AddDirectSubclass(const Class& subclass) const {
 }
 
 
+void Class::ClearDirectSubclasses() const {
+  StorePointer(&raw_ptr()->direct_subclasses_, GrowableObjectArray::null());
+}
+
+
 RawArray* Class::constants() const {
   return raw_ptr()->constants_;
 }
@@ -9694,6 +9699,12 @@ bool Library::ImportsCorelib() const {
 }
 
 
+void Library::DropDependencies() const {
+  StorePointer(&raw_ptr()->imports_, Array::null());
+  StorePointer(&raw_ptr()->exports_, Array::null());
+}
+
+
 void Library::AddImport(const Namespace& ns) const {
   Array& imports = Array::Handle(this->imports());
   intptr_t capacity = imports.Length();