Destructuring consistency fixes (#1417)

- Use AST_Destructuring for lhf assignment patterns
- Use AST_DefaultAssign for default assignments
- Add more checks for lhs expressions
- Add lots of testing
- Cleanup ast (e.g. remove default property)
- Fix #1402 based on a patch from @kzc
- Refine spread allowance in array destructring pattern
- Add destructuring AST tree checker

lib/compress.js

@@ -74,6 +74,7 @@ function Compressor(options, false_by_default) {
         pure_funcs    : null,
         negate_iife   : !false_by_default,
         screw_ie8     : true,
+        ecma          : 5,
         drop_console  : false,
         angular       : false,
         warnings      : true,
@@ -2317,7 +2318,7 @@ merge(Compressor.prototype, {
                             if (!fun) return self;
                             var args = fun.argnames.map(function(arg, i){
                                 return make_node(AST_String, self.args[i], {
-                                    value: arg.print_to_string()
+                                    value: arg.print_to_string({ecma: compressor.option("ecma")})
                                 });
                             });
                             var code = OutputStream();
@@ -2886,6 +2887,22 @@ merge(Compressor.prototype, {
         return self;
     });
 
+    OPT(AST_DefaultAssign, function(self, compressor){
+        if (!compressor.option("evaluate")) {
+            return self;
+        }
+        var evaluateRight = self.right.evaluate(compressor);
+
+        // `[x = undefined] = foo` ---> `[x] = foo`
+        if (evaluateRight.length > 1 && evaluateRight[1] === undefined) {
+            self = self.left;
+        } else {
+            self.right = evaluateRight[0];
+        }
+
+        return self;
+    });
+
     OPT(AST_Conditional, function(self, compressor){
         if (!compressor.option("conditionals")) return self;
         if (self.condition instanceof AST_Seq) {