fix keyword shorthand property output for ecma >= 6 (#2493)

fixes #2491

test/compress/harmony.js

@@ -937,3 +937,41 @@ issue_2349b: {
     ]
     node_version: ">=7"
 }
+
+shorthand_keywords: {
+    beautify = {
+        ecma: 6,
+    }
+    input: {
+        var foo = 0,
+            async = 1,
+            await = 2,
+            implements = 3,
+            package = 4,
+            private = 5,
+            protected = 6,
+            static = 7,
+            yield = 8;
+        console.log({
+            foo: foo,
+            0: 0,
+            NaN: NaN,
+            async: async,
+            await: await,
+            false: false,
+            implements: implements,
+            null: null,
+            package: package,
+            private: private,
+            protected: protected,
+            static: static,
+            this: this,
+            true: true,
+            undefined: undefined,
+            yield: yield,
+        });
+    }
+    expect_exact: "var foo=0,async=1,await=2,implements=3,package=4,private=5,protected=6,static=7,yield=8;console.log({foo,0:0,NaN:NaN,async,await,false:false,implements:implements,null:null,package:package,private:private,protected:protected,static:static,this:this,true:true,undefined:void 0,yield});"
+    expect_stdout: true
+    node_version: ">=4"
+}