fix for-of loop with const iterator (#1899)

lib/parse.js

@@ -1785,7 +1785,7 @@ function parse($TEXT, options) {
                     name  : as_symbol(sym_type),
                     value : is("operator", "=")
                         ? (next(), expression(false, no_in))
-                        : kind === "const" && S.input.has_directive("use strict")
+                        : !no_in && kind === "const" && S.input.has_directive("use strict")
                             ? croak("Missing initializer in const declaration") : null,
                     end   : prev()
                 })
@@ -1814,10 +1814,10 @@ function parse($TEXT, options) {
         });
     };
 
-    var const_ = function() {
+    var const_ = function(no_in) {
         return new AST_Const({
             start       : prev(),
-            definitions : vardefs(false, "const"),
+            definitions : vardefs(no_in, "const"),
             end         : prev()
         });
     };

test/compress/harmony.js

@@ -392,3 +392,35 @@ format_methods: {
         "}",
     ]
 }
+
+issue_1898: {
+    options = {
+    }
+    mangle = {
+    }
+    input: {
+        class Foo {
+            bar() {
+                for (const x of [ 6, 5 ]) {
+                    for (let y of [ 4, 3 ]) {
+                        for (var z of [ 2, 1 ]) {
+                            console.log(x, y, z);
+                        }
+                    }
+                }
+            }
+        }
+        new Foo().bar();
+    }
+    expect: {
+        class Foo {
+            bar() {
+                for (const n of [ 6, 5 ])
+                    for (let r of [ 4, 3 ])
+                        for (var o of [ 2, 1 ])
+                            console.log(n, r, o);
+            }
+        }
+        new Foo().bar();
+    }
+}