improve usability of --mangle-props (#5669)

lib/ast.js

@@ -1519,7 +1519,7 @@ var AST_New = DEFNODE("New", null, {
 var AST_Sequence = DEFNODE("Sequence", "expressions", {
     $documentation: "A sequence expression (comma-separated expressions)",
     $propdoc: {
-        expressions: "[AST_Node*] array of expressions (at least two)"
+        expressions: "[AST_Node*] array of expressions (at least two)",
     },
     _equals: function(node) {
         return all_equals(this.expressions, node.expressions);
@@ -1568,8 +1568,11 @@ var AST_PropAccess = DEFNODE("PropAccess", "expression optional property termina
     },
 });
 
-var AST_Dot = DEFNODE("Dot", null, {
+var AST_Dot = DEFNODE("Dot", "quoted", {
     $documentation: "A dotted property access expression",
+    $propdoc: {
+        quoted: "[boolean] whether property is transformed from a quoted string",
+    },
     walk: function(visitor) {
         var node = this;
         visitor.visit(node, function() {
@@ -1618,7 +1621,7 @@ var AST_Unary = DEFNODE("Unary", "operator expression", {
     $documentation: "Base class for unary expressions",
     $propdoc: {
         operator: "[string] the operator",
-        expression: "[AST_Node] expression that this unary operator applies to"
+        expression: "[AST_Node] expression that this unary operator applies to",
     },
     _equals: function(node) {
         return this.operator == node.operator

lib/compress.js

@@ -13236,6 +13236,7 @@ Compressor.prototype.compress = function(node) {
                         optional: self.optional,
                         expression: expr,
                         property: property,
+                        quoted: true,
                     }).optimize(compressor);
                 }
             }

lib/minify.js

@@ -205,6 +205,7 @@ function minify(files, options) {
             toplevel.mangle_names(options.mangle);
         }
         if (timings) timings.properties = Date.now();
+        if (quoted_props) reserve_quoted_keys(toplevel, quoted_props);
         if (options.mangle && options.mangle.properties) mangle_properties(toplevel, options.mangle.properties);
         if (options.parse.expression) toplevel = toplevel.unwrap_expression();
         if (timings) timings.output = Date.now();

lib/output.js

@@ -1517,7 +1517,7 @@ function OutputStream(options) {
         var expr = self.expression;
         expr.print(output);
         var prop = self.property;
-        if (output.option("ie") && RESERVED_WORDS[prop]) {
+        if (output.option("ie") && RESERVED_WORDS[prop] || self.quoted && output.option("keep_quoted_props")) {
             if (self.optional) output.print("?.");
             output.with_square(function() {
                 output.add_mapping(self.end);

lib/propmangle.js

@@ -126,6 +126,8 @@ function reserve_quoted_keys(ast, reserved) {
     ast.walk(new TreeWalker(function(node) {
         if (node instanceof AST_ClassProperty) {
             if (node.start && node.start.quote) add(node.key);
+        } else if (node instanceof AST_Dot) {
+            if (node.quoted) add(node.property);
         } else if (node instanceof AST_ObjectProperty) {
             if (node.start && node.start.quote) add(node.key);
         } else if (node instanceof AST_Sub) {