fix invalid transform on const (#1919)

- preserve (re)assignment to `const` for runtime error
- suppress `cascade` on `const`, as runtime behaviour is ill-defined

lib/compress.js

@@ -440,6 +440,14 @@ merge(Compressor.prototype, {
         return fixed();
     });
 
+    function is_reference_const(ref) {
+        if (!(ref instanceof AST_SymbolRef)) return false;
+        var orig = ref.definition().orig;
+        for (var i = orig.length; --i >= 0;) {
+            if (orig[i] instanceof AST_SymbolConst) return true;
+        }
+    }
+
     function find_variable(compressor, name) {
         var scope, i = 0;
         while (scope = compressor.parent(i++)) {
@@ -1909,6 +1917,7 @@ merge(Compressor.prototype, {
                         && node instanceof AST_Assign
                         && node.operator == "="
                         && node.left instanceof AST_SymbolRef
+                        && !is_reference_const(node.left)
                         && scope === self) {
                         node.right.walk(tw);
                         return true;
@@ -3066,7 +3075,8 @@ merge(Compressor.prototype, {
             }
             if (left
                 && !(left instanceof AST_SymbolRef
-                    && left.definition().orig[0] instanceof AST_SymbolLambda)) {
+                    && (left.definition().orig[0] instanceof AST_SymbolLambda
+                        || is_reference_const(left)))) {
                 var parent, field;
                 var cdr = self.cdr;
                 while (true) {