1. Dynamic compute the main closure that needs to be run by the main isolate

runtime/bin/builtin.dart

 // 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.
 
 library builtin;
 
 // NOTE: Do not import 'dart:io' in builtin.
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_internal' hide Symbol;
 import 'dart:isolate';
 import 'dart:typed_data';
 
 // Embedder sets this to true if the --trace-loading flag was passed on the
 // command line.
 bool _traceLoading = false;
 
 // Before handling an embedder entrypoint we finalize the setup of the
 // dart:_builtin library.
 bool _setupCompleted = false;
 
-// The root library (aka the script) is imported into this library. The
-// standalone embedder uses this to lookup the main entrypoint in the
-// root library's namespace.
-Function _getMainClosure() => main;
-
 // 'print' implementation.
 // The standalone embedder registers the closurized _print function with the
 // dart:core library.
 void _printString(String s) native "Builtin_PrintString";
 
 void _print(arg) {
   _printString(arg.toString());
 }
 
 _getPrintClosure() => _print;
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     if (_traceLoading) {
       _log("Exception when resolving package URI: $packageUri");
     }
     result = null;
   }
   if (_traceLoading) {
     _log("Resolved '$packageUri' to '$result'");
   }
   return result;
 }