[vm] Fix link to learn more about missing main function

The previous link no longer lead to a relevant destination. While I can add a client-side redirect on dart.dev, this CL updates the link to a redirect we can better guarantee as up to date in the long term.

TEST=vm/cc/DartAPI_RootLibraryMissingMain

Change-Id: I1e9dbf7a32c1468c308e9cf00d96575ba3c24695
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373322
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/dart_api_impl.cc

@@ -5440,8 +5440,7 @@ DART_EXPORT Dart_Handle Dart_LoadScriptFromKernel(const uint8_t* buffer,
   if (tmp.IsNull()) {
     return Api::NewError(
         "Invoked Dart programs must have a 'main' function defined:\n"
-        "https://dart.dev/guides/language/"
-        "language-tour#a-basic-dart-program");
+        "https://dart.dev/to/main-function");
   }
   library ^= tmp.ptr();
   IG->object_store()->set_root_library(library);

runtime/vm/dart_api_impl_test.cc

@@ -7488,6 +7488,19 @@ TEST_CASE(DartAPI_RootLibrary) {
   EXPECT(Dart_IsNull(root_lib));  // Root library did change.
 }
 
+TEST_CASE(DartAPI_RootLibraryMissingMain) {
+  const char* kScriptChars =
+      "notMain() {"
+      "  return 12345;"
+      "}";
+
+  Dart_Handle result = LoadScript(TestCase::url(), kScriptChars);
+
+  EXPECT_ERROR(result,
+               "Invoked Dart programs must have a 'main' function defined:\n"
+               "https://dart.dev/to/main-function");
+}
+
 TEST_CASE(DartAPI_LookupLibrary) {
   const char* kScriptChars =
       "import 'library1_dart';"