Use Mix.shell().error() for compiler tasks warnings

Author: josevalim

lib/mix/lib/mix/tasks/compile.leex.ex

@@ -66,8 +66,8 @@ defmodule Mix.Tasks.Compile.Leex do
   defp preload(project) do
     # TODO: Remove me in Elixir v2.0
     unless :leex in List.wrap(project[:compilers]) do
-      IO.warn(
-        "in order to compile .xrl files, you must add \"compilers: [:leex] ++ Mix.compilers()\" to the \"def project\" section of #{project[:app]}'s mix.exs"
+      Mix.shell().error(
+        "warning: in order to compile .xrl files, you must add \"compilers: [:leex] ++ Mix.compilers()\" to the \"def project\" section of #{project[:app]}'s mix.exs"
       )
     end
 

lib/mix/lib/mix/tasks/compile.yecc.ex

@@ -67,8 +67,8 @@ defmodule Mix.Tasks.Compile.Yecc do
   defp preload(project) do
     # TODO: Remove me in Elixir v2.0
     unless :yecc in List.wrap(project[:compilers]) do
-      IO.warn(
-        "in order to compile .yrl files, you must add \"compilers: [:yecc] ++ Mix.compilers()\" to the \"def project\" section of #{project[:app]}'s mix.exs"
+      Mix.shell().error(
+        "warning: in order to compile .yrl files, you must add \"compilers: [:yecc] ++ Mix.compilers()\" to the \"def project\" section of #{project[:app]}'s mix.exs"
       )
     end