Harden a few builtins

src/macros.py

 # Copyright 2006-2009 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
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 # "IsCallable(O)"
 # We assume here that this is the same as being either a function or a function
 # proxy. That ignores host objects with [[Call]] methods, but in most situations
 # we cannot handle those anyway.
 macro IS_SPEC_FUNCTION(arg) = (%_ClassOf(arg) === 'Function');
 
 # Macro for ES6 CheckObjectCoercible
 # 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;
 const kBoundArgumentsStartIndex = 2;
 
 # Inline macros. Use %IS_VAR to make sure arg is evaluated only once.
 macro NUMBER_IS_NAN(arg) = (!%_IsSmi(%IS_VAR(arg)) && !(arg == arg));
 macro NUMBER_IS_FINITE(arg) = (%_IsSmi(%IS_VAR(arg)) || ((arg == arg) && (arg != 1/0) && (arg != -1/0)));
 macro TO_INTEGER(arg) = (%_IsSmi(%IS_VAR(arg)) ? arg : %NumberToInteger(ToNumber(arg)));
 macro TO_INTEGER_FOR_SIDE_EFFECT(arg) = (%_IsSmi(%IS_VAR(arg)) ? arg : ToNumber(arg));
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 # Matches PropertyAttributes from property-details.h
 const PROPERTY_ATTRIBUTES_NONE = 0;
 const PROPERTY_ATTRIBUTES_STRING = 8;
 const PROPERTY_ATTRIBUTES_SYMBOLIC = 16;
 const PROPERTY_ATTRIBUTES_PRIVATE_SYMBOL = 32;
 
 # Use for keys, values and entries iterators.
 const ITERATOR_KIND_KEYS = 1;
 const ITERATOR_KIND_VALUES = 2;
 const ITERATOR_KIND_ENTRIES = 3;