Node 6 support (#484)

This patch:
- introduces a transpiler which substitutes async/await logic with
generators.
- starts using the transpiler to generate a node6-compatible version of puppeteer
- introduces a runtime-check to decide which version of code to use

Fixes #316.

utils/doclint/check_public_api/ESTreeWalker.js

@@ -1,135 +0,0 @@
-// Copyright (c) 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-/**
- * @unrestricted
- */
-class ESTreeWalker {
-  /**
-   * @param {function(!ESTree.Node):(!Object|undefined)} beforeVisit
-   * @param {function(!ESTree.Node)=} afterVisit
-   */
-  constructor(beforeVisit, afterVisit) {
-    this._beforeVisit = beforeVisit;
-    this._afterVisit = afterVisit || new Function();
-  }
-
-  /**
-   * @param {!ESTree.Node} ast
-   */
-  walk(ast) {
-    this._innerWalk(ast, null);
-  }
-
-  /**
-   * @param {!ESTree.Node} node
-   * @param {?ESTree.Node} parent
-   */
-  _innerWalk(node, parent) {
-    if (!node)
-      return;
-    node.parent = parent;
-
-    if (this._beforeVisit.call(null, node) === ESTreeWalker.SkipSubtree) {
-      this._afterVisit.call(null, node);
-      return;
-    }
-
-    const walkOrder = ESTreeWalker._walkOrder[node.type];
-    if (!walkOrder)
-      return;
-
-    if (node.type === 'TemplateLiteral') {
-      const templateLiteral = /** @type {!ESTree.TemplateLiteralNode} */ (node);
-      const expressionsLength = templateLiteral.expressions.length;
-      for (let i = 0; i < expressionsLength; ++i) {
-        this._innerWalk(templateLiteral.quasis[i], templateLiteral);
-        this._innerWalk(templateLiteral.expressions[i], templateLiteral);
-      }
-      this._innerWalk(templateLiteral.quasis[expressionsLength], templateLiteral);
-    } else {
-      for (let i = 0; i < walkOrder.length; ++i) {
-        const entity = node[walkOrder[i]];
-        if (Array.isArray(entity))
-          this._walkArray(entity, node);
-        else
-          this._innerWalk(entity, node);
-      }
-    }
-
-    this._afterVisit.call(null, node);
-  }
-
-  /**
-   * @param {!Array.<!ESTree.Node>} nodeArray
-   * @param {?ESTree.Node} parentNode
-   */
-  _walkArray(nodeArray, parentNode) {
-    for (let i = 0; i < nodeArray.length; ++i)
-      this._innerWalk(nodeArray[i], parentNode);
-  }
-}
-
-/** @typedef {!Object} ESTreeWalker.SkipSubtree */
-ESTreeWalker.SkipSubtree = {};
-
-/** @enum {!Array.<string>} */
-ESTreeWalker._walkOrder = {
-  'AwaitExpression': ['arguments'],
-  'ArrayExpression': ['elements'],
-  'ArrowFunctionExpression': ['params', 'body'],
-  'AssignmentExpression': ['left', 'right'],
-  'BinaryExpression': ['left', 'right'],
-  'BlockStatement': ['body'],
-  'BreakStatement': ['label'],
-  'CallExpression': ['callee', 'arguments'],
-  'CatchClause': ['param', 'body'],
-  'ClassBody': ['body'],
-  'ClassDeclaration': ['id', 'superClass', 'body'],
-  'ClassExpression': ['id', 'superClass', 'body'],
-  'ConditionalExpression': ['test', 'consequent', 'alternate'],
-  'ContinueStatement': ['label'],
-  'DebuggerStatement': [],
-  'DoWhileStatement': ['body', 'test'],
-  'EmptyStatement': [],
-  'ExpressionStatement': ['expression'],
-  'ForInStatement': ['left', 'right', 'body'],
-  'ForOfStatement': ['left', 'right', 'body'],
-  'ForStatement': ['init', 'test', 'update', 'body'],
-  'FunctionDeclaration': ['id', 'params', 'body'],
-  'FunctionExpression': ['id', 'params', 'body'],
-  'Identifier': [],
-  'IfStatement': ['test', 'consequent', 'alternate'],
-  'LabeledStatement': ['label', 'body'],
-  'Literal': [],
-  'LogicalExpression': ['left', 'right'],
-  'MemberExpression': ['object', 'property'],
-  'MethodDefinition': ['key', 'value'],
-  'NewExpression': ['callee', 'arguments'],
-  'ObjectExpression': ['properties'],
-  'ObjectPattern': ['properties'],
-  'ParenthesizedExpression': ['expression'],
-  'Program': ['body'],
-  'Property': ['key', 'value'],
-  'ReturnStatement': ['argument'],
-  'SequenceExpression': ['expressions'],
-  'Super': [],
-  'SwitchCase': ['test', 'consequent'],
-  'SwitchStatement': ['discriminant', 'cases'],
-  'TaggedTemplateExpression': ['tag', 'quasi'],
-  'TemplateElement': [],
-  'TemplateLiteral': ['quasis', 'expressions'],
-  'ThisExpression': [],
-  'ThrowStatement': ['argument'],
-  'TryStatement': ['block', 'handler', 'finalizer'],
-  'UnaryExpression': ['argument'],
-  'UpdateExpression': ['argument'],
-  'VariableDeclaration': ['declarations'],
-  'VariableDeclarator': ['id', 'init'],
-  'WhileStatement': ['test', 'body'],
-  'WithStatement': ['object', 'body'],
-  'YieldExpression': ['argument']
-};
-
-module.exports = ESTreeWalker;

utils/doclint/check_public_api/JSBuilder.js

@@ -15,7 +15,7 @@
  */
 
 const esprima = require('esprima');
-const ESTreeWalker = require('./ESTreeWalker');
+const ESTreeWalker = require('../../ESTreeWalker');
 const Documentation = require('./Documentation');
 
 class JSOutline {