Fix issue 17514770 (https://b2.corp.google.com/issues/17514770)

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 40420)
+++ runtime/vm/object.h	(working copy)
@@ -2888,6 +2888,8 @@
     return raw() == CoreLibrary();
   }
 
+  inline intptr_t UrlHash() const;
+
   static RawLibrary* LookupLibrary(const String& url);
   static RawLibrary* GetLibrary(intptr_t index);
 
@@ -7452,6 +7454,13 @@
 }
 
 
+intptr_t Library::UrlHash() const {
+  intptr_t result = Smi::Value(url()->ptr()->hash_);
+  ASSERT(result != 0);
+  return result;
+}
+
+
 void MegamorphicCache::SetEntry(const Array& array,
                                 intptr_t index,
                                 const Smi& class_id,