Revert "remove support for const (#1910)"

This reverts commit c391576d52.

lib/parse.js

@@ -912,6 +912,9 @@ function parse($TEXT, options) {
               case "var":
                 return tmp = var_(), semicolon(), tmp;
 
+              case "const":
+                return tmp = const_(), semicolon(), tmp;
+
               case "with":
                 if (S.input.has_directive("use strict")) {
                     croak("Strict mode may not include a with statement");
@@ -1146,13 +1149,16 @@ function parse($TEXT, options) {
         });
     };
 
-    function vardefs(no_in) {
+    function vardefs(no_in, in_const) {
         var a = [];
         for (;;) {
             a.push(new AST_VarDef({
                 start : S.token,
-                name  : as_symbol(AST_SymbolVar),
-                value : is("operator", "=") ? (next(), expression(false, no_in)) : null,
+                name  : as_symbol(in_const ? AST_SymbolConst : AST_SymbolVar),
+                value : is("operator", "=")
+                    ? (next(), expression(false, no_in))
+                    : in_const && S.input.has_directive("use strict")
+                        ? croak("Missing initializer in const declaration") : null,
                 end   : prev()
             }));
             if (!is("punc", ","))
@@ -1165,7 +1171,15 @@ function parse($TEXT, options) {
     var var_ = function(no_in) {
         return new AST_Var({
             start       : prev(),
-            definitions : vardefs(no_in),
+            definitions : vardefs(no_in, false),
+            end         : prev()
+        });
+    };
+
+    var const_ = function() {
+        return new AST_Const({
+            start       : prev(),
+            definitions : vardefs(false, true),
             end         : prev()
         });
     };