Fix -O0 compilation errors

runtime/bin/loader.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 
 #include "bin/loader.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 #include "bin/extensions.h"
@@ skipping 649 matching lines @@
     char* part_uri = reinterpret_cast<char*>(malloc(len + 1));
     snprintf(part_uri, len + 1, "%s/%s", library_url_string, url_string);
     Dart_Handle part_uri_obj = DartUtils::NewString(part_uri);
     free(part_uri);
     return Dart_LoadSource(library,
                            part_uri_obj, Dart_Null(),
                            Builtin::PartSource(id, url_string), 0, 0);
   }
   // All cases should have been handled above.
   UNREACHABLE();
+  return Dart_Null();
 }
 
 
 void Loader::InitOnce() {
   loader_infos_lock_ = new Mutex();
 }
 
 
 Mutex* Loader::loader_infos_lock_;
 Loader::LoaderInfo* Loader::loader_infos_ = NULL;
@@ skipping 77 matching lines @@
   MutexLocker ml(loader_infos_lock_);
   Loader* loader = LoaderForLocked(dest_port_id);
   if (loader == NULL) {
     return;
   }
   loader->QueueMessage(message);
 }
 
 }  // namespace bin
 }  // namespace dart