Implement spawnUri from dart:isolate. This function allows us to

runtime/bin/builtin.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 <stdio.h>
 
 #include "include/dart_api.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
@@ skipping 63 matching lines @@
       url = Dart_NewString(DartUtils::kUriLibURL);
       break;
     case kUtfLibrary:
       url = Dart_NewString(DartUtils::kUtfLibURL);
       break;
     default:
       return Dart_Error("Unknown builtin library requested.");
   }
   Dart_Handle library = Dart_LookupLibrary(url);
   if (Dart_IsError(library)) {
-    Dart_Handle import_map = Dart_NewList(0);
-    library = Dart_LoadLibrary(url, Source(id), import_map);
+    library = Dart_LoadLibrary(url, Source(id));
     if (!Dart_IsError(library)) {
       SetupLibrary(library, id);
     }
   }
   DART_CHECK_VALID(library);
   return library;
 }
 
 
 void Builtin::ImportLibrary(Dart_Handle library, BuiltinLibraryId id) {
   Dart_Handle imported_library = LoadLibrary(id);
   // Import the library into current library.
   DART_CHECK_VALID(Dart_LibraryImportLibrary(library, imported_library));
 }