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

runtime/bin/main.cc

Index: runtime/bin/main.cc
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index 61d76f7a80bf32c58a635f0493746ab9ee7dc76b..948f112894e41e69405e4caf947c01b669d3d6fe 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -1487,7 +1487,6 @@ bool RunMainIsolate(const char* script_name, CommandLineOptions* dart_options) {
 
     if (gen_snapshot_kind == kAppAOT) {
       Dart_QualifiedFunctionName standalone_entry_points[] = {
-          {"dart:_builtin", "::", "_getMainClosure"},
           {"dart:_builtin", "::", "_getPrintClosure"},
           {"dart:_builtin", "::", "_getUriBaseClosure"},
           {"dart:_builtin", "::", "_libraryFilePath"},
@@ -1571,13 +1570,16 @@ bool RunMainIsolate(const char* script_name, CommandLineOptions* dart_options) {
         CHECK_RESULT(result);
       }
 
-      // The helper function _getMainClosure creates a closure for the main
-      // entry point which is either explicitly or implictly exported from the
-      // root library.
+      // Create a closure for the main entry point which is in the exported
+      // namespace of the root library or invoke a getter of the same name
+      // in the exported namespace and return the resulting closure.
       Dart_Handle main_closure =
-          Dart_Invoke(isolate_data->builtin_lib(),
-                      Dart_NewStringFromCString("_getMainClosure"), 0, NULL);
+          Dart_GetClosure(root_lib, Dart_NewStringFromCString("main"));
       CHECK_RESULT(main_closure);
+      if (!Dart_IsClosure(main_closure)) {
+        ErrorExit(kErrorExitCode,
+                  "Unable to find 'main' in root library '%s'\n", script_name);
+      }
 
       // Call _startIsolate in the isolate library to enable dispatching the
       // initial startup message.