Implement new Function.prototype.toString and fix CreateDynamicFunction parsing

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index 297b1db94be1cfb68ca7171ded7ea37277fe87fd..35ade518f3de7bfb97fa54a76f98177386e930f1 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -582,6 +582,7 @@ class ErrorUtils : public AllStatic {
     "Setter function argument must not be a rest parameter")                   \
   T(ParamDupe, "Duplicate parameter name not allowed in this context")         \
   T(ParenthesisInArgString, "Function arg string contains parenthesis")        \
+  T(ExpectedEndOfParameters, "Expected arg string to end here")                \

[Dan Ehrenberg, 2016/12/06 00:32:13]

Aren't we talking about cases where someone effectively uses invalid syntax in
parameters for the function constructor? The exception text could reflect that
more directly.

[jwolfe, 2017/01/13 00:28:47]

I split the error into two different messages:

(function anonymous() { function f(
                    ^
SyntaxError: Arg string terminates parameters early

and:

(function anonymous(a=function(
                              ^^
SyntaxError: Unexpected end of arg string

I'm open to feedback on making these errors more clear.

   T(RuntimeWrongNumArgs, "Runtime function given wrong number of arguments")   \
   T(SingleFunctionLiteral, "Single function literal required")                 \
   T(SloppyFunction,                                                            \