Fail more helpfully when invoking a null native function.

runtime/vm/native_entry.cc

Index: runtime/vm/native_entry.cc
diff --git a/runtime/vm/native_entry.cc b/runtime/vm/native_entry.cc
index f89c9a050fa644f458d693554e1f66214d6f85e7..c7e194b7726c6952af1484707ec8124717fdb5bf 100644
--- a/runtime/vm/native_entry.cc
+++ b/runtime/vm/native_entry.cc
@@ -39,7 +39,7 @@ NativeFunction NativeEntry::ResolveNative(const Library& library,
                                           int number_of_arguments,
                                           bool* auto_setup_scope) {
   // Now resolve the native function to the corresponding native entrypoint.
-  if (library.native_entry_resolver() == 0) {
+  if (library.native_entry_resolver() == NULL) {
     // Native methods are not allowed in the library to which this
     // class belongs in.
     return NULL;
@@ -62,7 +62,7 @@ const uint8_t* NativeEntry::ResolveSymbolInLibrary(const Library& library,
                                                    uword pc) {
   Dart_NativeEntrySymbol symbol_resolver =
       library.native_entry_symbol_resolver();
-  if (symbol_resolver == 0) {
+  if (symbol_resolver == NULL) {
     // Cannot reverse lookup native entries.
     return NULL;
   }
@@ -232,8 +232,13 @@ static NativeFunction ResolveNativeFunction(Zone* zone,
   ASSERT(!native_name.IsNull());
 
   const int num_params = NativeArguments::ParameterCountForResolution(func);
-  return NativeEntry::ResolveNative(library, native_name, num_params,
-                                    is_auto_scope);
+  NativeFunction native_function = NativeEntry::ResolveNative(
+      library, native_name, num_params, is_auto_scope);
+  if (native_function == NULL) {
+    FATAL2("Failed to resolve native function '%s' in '%s'\n",
+           native_name.ToCString(), func.ToQualifiedCString());
+  }
+  return native_function;
 }