Here's a template for how I plan on doing error-checking on inputs for

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
===================================================================
--- runtime/vm/dart_api_impl.cc	(revision 1375)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -27,6 +27,23 @@
 
 namespace dart {
 
+#define UNWRAP_NONNULL(dart_handle, vm_handle, Type)                    \

[Ivan Posva, 2011/11/10 00:12:55]

How about adding a line like this here:

Type& vm_handle = Type::Handle();

This would avoid having to duplicating the handle allocation everywhere.

[turnidge, 2011/11/10 17:48:57]

I tried this and it was a bit too magical for me.  I like having the handle
declaration outside of the macro to make it easier to guess what is going on. 
Okay?

On 2011/11/10 00:12:55, Ivan Posva wrote:

+  do {                                                                  \
+    const Object& tmp = Object::Handle(Api::UnwrapHandle((dart_handle))); \
+    if (tmp.Is##Type()) {                                               \
+      (vm_handle) ^= tmp.raw();                                         \
+    } else if (tmp.IsNull()) {                                          \
+      return Api::Error("%s expects argument '%s' to be non-null.",     \
+                        __func__, #dart_handle);                        \
+    } else if (tmp.IsApiFailure()) {                                    \
+      return dart_handle;                                               \
+    } else {                                                            \
+      return Api::Error("%s expects argument '%s' to be of type %s.",   \
+                        __func__, #dart_handle, #Type);                 \
+    }                                                                   \

[Ivan Posva, 2011/11/10 00:12:55]

\ at column 80.

[turnidge, 2011/11/10 17:48:57]

Done.

+  } while (0)
+
+
 DART_EXPORT bool Dart_IsValid(const Dart_Handle& handle) {
   ASSERT(Isolate::Current() != NULL);
   Zone zone;  // Setup a VM zone as we are creating some handles.
@@ -381,10 +398,12 @@
 DART_EXPORT Dart_Handle Dart_LookupLibrary(Dart_Handle url) {
   Zone zone;  // Setup a VM zone as we are creating some handles.
   HandleScope scope;  // Setup a VM handle scope.
-  const String& url_str = String::CheckedHandle(Api::UnwrapHandle(url));
+  String& url_str = String::Handle();
+  UNWRAP_NONNULL(url, url_str, String);
   const Library& library = Library::Handle(Library::LookupLibrary(url_str));
   if (library.IsNull()) {
-    return Api::Error("Unknown library");
+    return Api::Error("%s: library '%s' not found.",
+                      __func__, url_str.ToCString());
   } else {
     return Api::NewLocalHandle(library);
   }