Strawman: check strong mode free variables against the global object.

src/messages.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // -------------------------------------------------------------------
 
 var kMessages = {
   // Error
   cyclic_proto:                  ["Cyclic __proto__ value"],
   code_gen_from_strings:         ["%0"],
@@ skipping 154 matching lines @@
   strict_poison_pill:            ["'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them"],
   strict_caller:                 ["Illegal access to a strict mode caller function."],
   strong_ellision:               ["In strong mode, arrays with holes are deprecated, use maps instead"],
   strong_arguments:              ["In strong mode, 'arguments' is deprecated, use '...args' instead"],
   strong_equal:                  ["In strong mode, '==' and '!=' are deprecated, use '===' and '!==' instead"],
   strong_delete:                 ["In strong mode, 'delete' is deprecated, use maps or sets instead"],
   strong_var:                    ["In strong mode, 'var' is deprecated, use 'let' or 'const' instead"],
   strong_for_in:                 ["In strong mode, 'for'-'in' loops are deprecated, use 'for'-'of' instead"],
   strong_empty:                  ["In strong mode, empty sub-statements are deprecated, make them explicit with '{}' instead"],
   strong_use_before_declaration: ["In strong mode, declaring variable '", "%0", "' before its use is required"],
+  strong_unbound_global:         ["In strong mode, using an undeclared global variable is not allowed"],
   strong_super_call_missing:     ["In strong mode, invoking the super constructor in a subclass is required"],
   strong_super_call_duplicate:   ["In strong mode, invoking the super constructor multiple times is deprecated"],
   strong_super_call_nested:      ["In strong mode, invoking the super constructor nested inside another statement or expression is deprecated"],
   strong_constructor_return_value: ["In strong mode, returning a value from a constructor is deprecated"],
   strong_constructor_return_misplaced: ["In strong mode, returning from a constructor before its super constructor invocation is deprecated"],
   sloppy_lexical:                ["Block-scoped declarations (let, const, function, class) not yet supported outside strict mode"],
   malformed_arrow_function_parameter_list: ["Malformed arrow function parameter list"],
   generator_poison_pill:         ["'caller' and 'arguments' properties may not be accessed on generator functions."],
   cant_prevent_ext_external_array_elements: ["Cannot prevent extension of an object with external array elements"],
   redef_external_array_element:  ["Cannot redefine a property of an object with external array elements"],
@@ skipping 1142 matching lines @@
 function SetUpStackOverflowBoilerplate() {
   var boilerplate = MakeRangeError('stack_overflow', []);
 
   %DefineAccessorPropertyUnchecked(
       boilerplate, 'stack', StackTraceGetter, StackTraceSetter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();