fix comments output & improve /*@__PURE__*/

- fix whitespace around comments
- fix comment parsing around parentheses
- consider parentheses when parsing `/*@__PURE__*/`
- remove all `/*@__PURE__*/` on output

fixes #2638

test/mocha/minify.js

@@ -247,7 +247,7 @@ describe("minify", function() {
             var code = result.code;
             assert.strictEqual(code, "//  comment1   comment2\nbar();");
         });
-        it("should not drop #__PURE__ hint if function is retained", function() {
+        it("should drop #__PURE__ hint if function is retained", function() {
             var result = Uglify.minify("var a = /*#__PURE__*/(function(){ foo(); })();", {
                 output: {
                     comments: "all",
@@ -255,7 +255,7 @@ describe("minify", function() {
                 }
             });
             var code = result.code;
-            assert.strictEqual(code, "var a=/*#__PURE__*/function(){foo()}();");
+            assert.strictEqual(code, "var a=/* */function(){foo()}();");
         })
     });