Convert p: function(){} to p(){} in object literals (#2199)

when `compress` option `ecma` is 6 or greater.

lib/compress.js

@@ -4655,4 +4655,41 @@ merge(Compressor.prototype, {
     OPT(AST_PrefixedTemplateString, function(self, compressor){
         return self;
     });
+
+    OPT(AST_ObjectKeyVal, function(self, compressor){
+        // p:function(){} ---> p(){}
+        // p:function*(){} ---> *p(){}
+        // p:async function(){} ---> async p(){}
+        // p:()=>{} ---> p(){}
+        // p:async()=>{} ---> async p(){}
+        if (compressor.option("ecma") >= 6) {
+            var key = self.key;
+            var value = self.value;
+            var is_arrow_with_block = value instanceof AST_Arrow
+                && Array.isArray(value.body)
+                && !contains_this(value);
+            if ((is_arrow_with_block || value instanceof AST_Function) && !value.name) {
+                return make_node(AST_ConciseMethod, self, {
+                    async: value.async,
+                    is_generator: value.is_generator,
+                    key: key instanceof AST_Node ? key : make_node(AST_SymbolMethod, self, {
+                        name: key,
+                    }),
+                    value: make_node(AST_Accessor, value, value),
+                });
+            }
+        }
+        return self;
+
+        function contains_this(node) {
+            var result;
+            var tw = new TreeWalker(function(node) {
+                if (node instanceof AST_This) {
+                    return result = true;
+                }
+            });
+            node.walk(tw);
+            return result;
+        }
+    });
 })();