Fix for issue 23908 - error message reports the exact error when a native extension fails to load.

runtime/bin/extensions_win.cc

Index: runtime/bin/extensions_win.cc
diff --git a/runtime/bin/extensions_win.cc b/runtime/bin/extensions_win.cc
index 8c0a15fb5c32619a467cd5755bc6240fb80b97cf..dc4c2f1e9fc9eae8073cdeed6b705d04973c1f35 100644
--- a/runtime/bin/extensions_win.cc
+++ b/runtime/bin/extensions_win.cc
@@ -16,12 +16,28 @@ namespace bin {
 const char* kPrecompiledLibraryName = "precompiled.dll";
 const char* kPrecompiledSymbolName = "_kInstructionsSnapshot";
 
-void* Extensions::LoadExtensionLibrary(const char* library_file) {
-  return LoadLibraryW(StringUtilsWin::Utf8ToWide(library_file));
+Dart_Handle Extensions::LoadExtensionLibrary(const char* library_file,
+                                             void** library_handle) {
+  ASSERT(library_handle != NULL);
+  *library_handle = LoadLibraryW(StringUtilsWin::Utf8ToWide(library_file));
+  if (*library_handle == NULL) {
+    OSError err;
+    return Dart_NewApiError(err.message());
+  }
+  return Dart_Null();
 }
 
-void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
-  return GetProcAddress(reinterpret_cast<HMODULE>(lib_handle), symbol);
+Dart_Handle Extensions::ResolveSymbol(void* lib_handle,
+                                      const char* symbol,
+                                      void** init_function) {
+  ASSERT(init_function != NULL);
+  *init_function = GetProcAddress(
+      reinterpret_cast<HMODULE>(lib_handle), symbol);
+  if (*init_function == NULL) {
+    OSError err;
+    return Dart_NewApiError(err.message());
+  }
+  return Dart_Null();
 }
 
 }  // namespace bin