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 71cf46c80aa6abf0e4c7f05e21e3daf16ff94b0a..3e53c376d048a6f9d0dd643a8fae92cc16464f7f 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -3507,6 +3507,11 @@ 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_;
 }
@@ -9720,6 +9725,12 @@ 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();