Add type assertion to Symbol method. Fix JuliaLang#242 (JuliaLang#243)

* Add type assertion to `Symbol` method. Fix JuliaLang#242

* Remove comprehension type inference test

Author: cstjean

src/Compat.jl

@@ -397,7 +397,7 @@ if VERSION < v"0.4.0-dev+3732"
         calltypes[k] = v
     end
 elseif VERSION < v"0.5.0-dev+3831"
-    Base.Symbol(args...) = symbol(args...)
+    Base.Symbol(args...) = symbol(args...)::Symbol
 end
 
 if VERSION < v"0.5.0-dev+2396"