Start using strict mode for everything

V8 engineer said it is always faster, so that’s probably reliable info
:)

Setup.hs

@@ -148,7 +148,8 @@ buildRuntime :: LocalBuildInfo -> [FilePath] -> IO ()
 buildRuntime lbi elmos = do
   createDirectoryIfMissing True (rtsDir lbi)
   let rts' = rts lbi
-  writeFile rts' "var Elm = {}; Elm.Native = {}; Elm.Native.Graphics = {};\n\
+  writeFile rts' "'use strict';\n\
+                 \var Elm = {}; Elm.Native = {}; Elm.Native.Graphics = {};\n\
                  \var ElmRuntime = {}; ElmRuntime.Render = {};\n"
   mapM_ (appendTo rts') =<< getFiles ".js" "libraries"
   mapM_ (appendTo rts') elmos

compiler/Generate/JavaScript.hs

@@ -293,7 +293,8 @@ generate unsafeModule =
     thisModule = dotSep ("_elm" : names modul ++ ["values"])
     programStmts =
         concat
-        [ setup (Just "_elm") (names modul ++ ["values"])
+        [ [ ExprStmt () $ string "use strict" ]
+        , setup (Just "_elm") (names modul ++ ["values"])
         , [ IfSingleStmt () thisModule (ret thisModule) ]
         , [ internalImports (List.intercalate "." (names modul)) ]
         , concatMap jsImport . Set.toList . Set.fromList . map fst $ imports modul