Harden a few builtins

src/macros.py

Index: src/macros.py
diff --git a/src/macros.py b/src/macros.py
index 39881fe76317401b93698e358f5b0d922db5bbc4..f402b3c0f30667c956ce0af79b8f89f9660c1c55 100644
--- a/src/macros.py
+++ b/src/macros.py
@@ -147,6 +147,10 @@ macro IS_SPEC_FUNCTION(arg) = (%_ClassOf(arg) === 'Function');
 # Will throw a TypeError of the form "[functionName] called on null or undefined".
 macro CHECK_OBJECT_COERCIBLE(arg, functionName) = if (IS_NULL_OR_UNDEFINED(arg) && !IS_UNDETECTABLE(arg)) throw MakeTypeError('called_on_null_or_undefined', [functionName]);
 
+# Equivalent of RUNTIME_ASSERT. Used to check for error conditions that should
+# not be triggerable from user code (just tests).
+macro BUILTIN_ASSERT(condition) = if (!(condition)) throw MakeInternalError();
+
 # Indices in bound function info retrieved by %BoundFunctionGetBindings(...).
 const kBoundFunctionIndex = 0;
 const kBoundThisIndex = 1;