[strong] Check constructor returns

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 5c5e2075c151c91dcd97ac722b866b92d41947c0..e46979acf8fc9ee66870e86810804ce814bb4cca 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -173,6 +173,8 @@ var kMessages = {
   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."],