[strong] Check constructor returns

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 155 matching lines @@
   strong_arguments:              ["Please don't use 'arguments' in strong mode, use '...args' instead"],
   strong_equal:                  ["Please don't use '==' or '!=' in strong mode, use '===' or '!==' instead"],
   strong_delete:                 ["Please don't use 'delete' in strong mode, use maps or sets instead"],
   strong_var:                    ["Please don't use 'var' in strong mode, use 'let' or 'const' instead"],
   strong_for_in:                 ["Please don't use 'for'-'in' loops in strong mode, use 'for'-'of' instead"],
   strong_empty:                  ["Please don't use empty sub-statements in strong mode, make them explicit with '{}' instead"],
   strong_use_before_declaration: ["Please declare variable '", "%0", "' before use in strong mode"],
   strong_super_call_missing:     ["Please always invoke the super constructor in subclasses in strong mode"],
   strong_super_call_duplicate:   ["Please don't invoke the super constructor multiple times in strong mode"],
   strong_super_call_nested:      ["Please don't invoke the super constructor nested inside another statement or expression in strong mode"],
+  strong_constructor_return_value: ["Please do not return a value from a constructor in strong mode"],

[Dmitry Lomov (no reviews), 2015/03/19 13:59:41]

Why do we keep pleading with users here?

[adamk, 2015/03/19 14:04:11]

Drive-by +1 here. The "please" sounds odd for something that's a hard
requirement of the language. That is, it reads more like a deprecation message
than an error.

[rossberg, 2015/03/19 16:02:57]

Strong mode intentionally has the mostly friendly semantics in the world. :)

Alright, these should be reformulated. But rather consistently in a separate CL.

+  strong_constructor_return_misplaced: ["Please do not return from a constructor before its super constructor invocation in strong mode"],
   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"],
   const_assign:                  ["Assignment to constant variable."],
   symbol_to_string:              ["Cannot convert a Symbol value to a string"],
   symbol_to_primitive:           ["Cannot convert a Symbol wrapper object to a primitive value"],
   symbol_to_number:              ["Cannot convert a Symbol value to a number"],
   module_export_undefined:       ["Export '", "%0", "' is not defined in module"],
@@ skipping 1137 matching lines @@
 function SetUpStackOverflowBoilerplate() {
   var boilerplate = MakeRangeError('stack_overflow', []);
 
   %DefineAccessorPropertyUnchecked(
       boilerplate, 'stack', StackTraceGetter, StackTraceSetter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();