Fix corner case in partial script url matching

runtime/vm/object.cc

 // Copyright (c) 2012, 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 "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 9094 matching lines @@
     const Array& scripts_array = Array::Handle(Array::MakeArray(scripts));
     StorePointer(&raw_ptr()->loaded_scripts_, scripts_array.raw());
   }
   return loaded_scripts();
 }
 
 
 // TODO(hausner): we might want to add a script dictionary to the
 // library class to make this lookup faster.
 RawScript* Library::LookupScript(const String& url) const {
+  const intptr_t url_length = url.Length();
+  if (url_length == 0) {
+    return Script::null();
+  }
   const Array& scripts = Array::Handle(LoadedScripts());
   Script& script = Script::Handle();
   String& script_url = String::Handle();
-  const intptr_t url_length = url.Length();
   const intptr_t num_scripts = scripts.Length();
   for (int i = 0; i < num_scripts; i++) {
     script ^= scripts.At(i);
     script_url = script.url();
     const intptr_t start_idx = script_url.Length() - url_length;
     if ((start_idx == 0) && url.Equals(script_url)) {
       return script.raw();
     } else if (start_idx > 0) {
       // If we do a suffix match, only match if the partial path
       // starts at or immediately after the path separator.
@@ skipping 11156 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart