[d8] Treat .mjs files as modules

This patch makes `d8` recognize files with the `.mjs` extension as
modules instead of classic scripts. This change can be tested by saving
the following JavaScript program as both `module.mjs` and as
`script.js`:

    console.log(this === undefined ? 'strict' : 'sloppy');

Then, run these files in `d8` without passing the `--module` flag:

    $ d8 module.mjs
    strict

    $ d8 script.js
    sloppy

The use of `.mjs` matches not just Google’s recommendation [1] but also
the current modules implementation in Node.js [2].

[1] https://developers.google.com/web/fundamentals/primers/modules
[2] https://nodejs.org/api/esm.html

Bug: v8:7950
Change-Id: I8f39420dc24a5eedd7e88d3b1aa48207ebfeff6e
Reviewed-on: https://chromium-review.googlesource.com/1140314
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54502}

src/d8.cc

@@ -2423,6 +2423,14 @@ SourceGroup::~SourceGroup() {
   thread_ = nullptr;
 }
 
+bool ends_with(const char* input, const char* suffix) {
+  size_t input_length = strlen(input);
+  size_t suffix_length = strlen(suffix);
+  if (suffix_length <= input_length) {
+    return strcmp(input + input_length - suffix_length, suffix) == 0;
+  }
+  return false;
+}
 
 void SourceGroup::Execute(Isolate* isolate) {
   bool exception_was_thrown = false;
@@ -2446,6 +2454,13 @@ void SourceGroup::Execute(Isolate* isolate) {
       }
       ++i;
       continue;
+    } else if (ends_with(arg, ".mjs")) {
+      Shell::options.script_executed = true;
+      if (!Shell::ExecuteModule(isolate, arg)) {
+        exception_was_thrown = true;
+        break;
+      }
+      continue;
     } else if (strcmp(arg, "--module") == 0 && i + 1 < end_offset_) {
       // Treat the next file as a module.
       arg = argv_[++i];
@@ -2905,8 +2920,8 @@ bool Shell::SetOptions(int argc, char* argv[]) {
       current->Begin(argv, i + 1);
     } else if (strcmp(str, "--module") == 0) {
       // Pass on to SourceGroup, which understands this option.
-    } else if (strncmp(argv[i], "--", 2) == 0) {
-      printf("Warning: unknown flag %s.\nTry --help for options\n", argv[i]);
+    } else if (strncmp(str, "--", 2) == 0) {
+      printf("Warning: unknown flag %s.\nTry --help for options\n", str);
     } else if (strcmp(str, "-e") == 0 && i + 1 < argc) {
       options.script_executed = true;
     } else if (strncmp(str, "-", 1) != 0) {

src/flags.cc

@@ -638,6 +638,7 @@ void FlagList::PrintHelp() {
         "  -e        execute a string in V8\n"
         "  --shell   run an interactive JavaScript shell\n"
         "  --module  execute a file as a JavaScript module\n\n"
+        "Note: the --module option is implicitly enabled for *.mjs files.\n\n"
         "Options:\n";
 
   for (const Flag& f : flags) {