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

runtime/vm/dart_api_impl.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "include/dart_api.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/dart.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_api_state.h"
 #include "vm/dart_entry.h"
 #include "vm/debuginfo.h"
 #include "vm/exceptions.h"
 #include "vm/growable_array.h"
 #include "vm/longjump.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/port.h"
 #include "vm/resolver.h"
 #include "vm/snapshot.h"
 #include "vm/stack_frame.h"
 #include "vm/timer.h"
 #include "vm/verifier.h"
 
 namespace dart {
 
+#define UNWRAP_NONNULL(dart_handle, vm_handle, Type)                          \
+  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);                       \
+    }                                                                         \
+  } 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.
   HandleScope scope;  // Setup a VM handle scope.
 
   // Make sure that the object isn't an ApiFailure.
   const Object& obj = Object::Handle(Api::UnwrapHandle(handle));
   return !obj.IsApiFailure();
 }
 
@@ skipping 334 matching lines @@
   const Library& import =
       Library::CheckedHandle(Api::UnwrapHandle(import_in));
   library.AddImport(import);
   return Api::Success();
 }
 
 
 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);
   }
 }
 
 
 DART_EXPORT Dart_Handle Dart_LoadLibrary(Dart_Handle url, Dart_Handle source) {
   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));
@@ skipping 1624 matching lines @@
   ASSERT(state != NULL);
   PersistentHandle* true_handle = state->True();
   return reinterpret_cast<Dart_Handle>(true_handle);
 }
 
 
 Dart_Handle Api::VError(const char* format, va_list args) {
   Zone zone;  // Setup a VM zone as we are creating some handles.
   HandleScope scope;  // Setup a VM handle scope.
 
-  intptr_t len = OS::VSNPrint(NULL, 0, format, args);
+  va_list args_copy;
+  va_copy(args_copy, args);
+  intptr_t len = OS::VSNPrint(NULL, 0, format, args_copy);
+  va_end(args_copy);
+
   char* buffer = reinterpret_cast<char*>(zone.Allocate(len + 1));
   OS::VSNPrint(buffer, (len + 1), format, args);
 
   const String& message = String::Handle(String::New(buffer));
   const Object& obj = Object::Handle(ApiFailure::New(message));
   return Api::NewLocalHandle(obj);
 }
 
-
 Dart_Handle Api::Error(const char* format, ...) {
   va_list args;
   va_start(args, format);
   Dart_Handle error = Api::VError(format, args);
   va_end(args);
   return error;
 }
 
 
 Dart_Handle Api::Null() {
@@ skipping 42 matching lines @@
   ASSERT(isolate != NULL);
   ApiState* state = isolate->api_state();
   ASSERT(state != NULL);
   ApiLocalScope* scope = state->top_scope();
   ASSERT(scope != NULL);
   return scope->zone().Reallocate(ptr, old_size, new_size);
 }
 
 
 }  // namespace dart