src: allow specifying entry point script

Introduce a `--js-entry-point=` CLI option that allows pointing
the node executable to a main entry point that’s not
`lib/internal/bootstrap_node.js`.

src/node.cc

@@ -167,6 +167,7 @@ static const char* icu_data_dir = nullptr;
 #endif
 
 const char* internal_modules_source_dir = nullptr;
+const char* js_entry_point = nullptr;
 
 // used by C++ modules as well
 bool no_deprecation = false;
@@ -3762,6 +3763,8 @@ static void ParseArgs(int* argc,
 #endif
     } else if (strncmp(arg, "--internal-modules-source-dir=", 30) == 0) {
       internal_modules_source_dir = arg + 30;
+    } else if (strncmp(arg, "--js-entry-point=", 17) == 0) {
+      js_entry_point = arg + 17;
     } else if (strcmp(arg, "--expose-internals") == 0 ||
                strcmp(arg, "--expose_internals") == 0) {
       // consumed in js

src/node_internals.h

@@ -47,6 +47,9 @@ extern bool v8_initialized;
 // Source file directory to load instead of the ones baked into the executable
 // for hacking.
 extern const char* internal_modules_source_dir;
+// Source file to load instead of the `bootstrap_node.js` one baked into the
+// executable.
+extern const char* js_entry_point;
 
 // Forward declaration
 class Environment;

src/node_javascript.cc

@@ -40,6 +40,10 @@ static MaybeLocal<String> MaybeLoadSourceFromDisk(Environment* env,
 }
 
 Local<String> MainSource(Environment* env) {
+  if (js_entry_point != nullptr) {
+    return InternalModuleReadFile(env, js_entry_point).ToLocalChecked();
+  }
+
   auto maybe_disk_src =
       MaybeLoadSourceFromDisk(env,
                               internal_bootstrap_node_name,

test/parallel/test-internal-modules-src-dir.js

@@ -22,3 +22,11 @@ assert.strictEqual(
     child_process.spawnSync(process.execPath,
                             [`--internal-modules-source-dir=${bootstrap_node}`,
                              '-p', '0']).status);
+
+const entry = path.join(bootstrap_node, 'internal/bootstrap_node.js');
+
+assert.strictEqual(
+    42,
+    child_process.spawnSync(process.execPath,
+                            [`--js-entry-point=${entry}`,
+                             '-p', '0']).status);