Use Object.create where appropriate

This breaks support on ancient Node and IE versions, but at this point
that's okay (IE11 has Object.create).

Closes #992

acorn-walk/src/index.js

@@ -164,17 +164,10 @@ export function findNodeBefore(node, pos, test, baseVisitor, state) {
   return max
 }
 
-// Fallback to an Object.create polyfill for older environments.
-const create = Object.create || function(proto) {
-  function Ctor() {}
-  Ctor.prototype = proto
-  return new Ctor
-}
-
 // Used to create a custom walker. Will fill in all missing node
 // type properties with the defaults.
 export function make(funcs, baseVisitor) {
-  let visitor = create(baseVisitor || base)
+  let visitor = Object.create(baseVisitor || base)
   for (let type in funcs) visitor[type] = funcs[type]
   return visitor
 }

acorn/src/expression.js

@@ -924,7 +924,7 @@ pp.isSimpleParamList = function(params) {
 // or "arguments" and duplicate parameters.
 
 pp.checkParams = function(node, allowDuplicates) {
-  let nameHash = {}
+  let nameHash = Object.create(null)
   for (let param of node.params)
     this.checkLValInnerPattern(param, BIND_VAR, allowDuplicates ? null : nameHash)
 }

acorn/src/state.js

@@ -71,7 +71,7 @@ export class Parser {
     // Labels in scope.
     this.labels = []
     // Thus-far undefined exports.
-    this.undefinedExports = {}
+    this.undefinedExports = Object.create(null)
 
     // If enabled, skip leading hashbang line.
     if (this.pos === 0 && options.allowHashBang && this.input.slice(0, 2) === "#!")