[doclint] Prepare doclint for more checks

This patch refactors doclint so that more checks and more generators
could be added.

This patch:
- Introduces 'Source' class, which holds file content in-memory and
  allows it to be updated.
- Introduces 'Message' class - which is a pair of a text and a type.
  Messages could have either 'error' type or 'warning' type.

utils/doclint/SourceFactory.js

@@ -0,0 +1,131 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+const path = require('path');
+const fs = require('fs');
+const {promisify} = require('util');
+const readFileAsync = promisify(fs.readFile);
+const readdirAsync = promisify(fs.readdir);
+const writeFileAsync = promisify(fs.writeFile);
+
+const PROJECT_DIR = path.join(__dirname, '..', '..');
+
+class Source {
+  /**
+   * @param {string} filePath
+   * @param {string} text
+   */
+  constructor(filePath, text) {
+    this._filePath = filePath;
+    this._projectPath = path.relative(PROJECT_DIR, filePath);
+    this._name = path.basename(filePath);
+    this._text = text;
+    this._hasUpdatedText = false;
+  }
+
+  /**
+   * @return {string}
+   */
+  filePath() {
+    return this._filePath;
+  }
+
+  /**
+   * @return {string}
+   */
+  projectPath() {
+    return this._projectPath;
+  }
+
+  /**
+   * @return {string}
+   */
+  name() {
+    return this._name;
+  }
+
+  /**
+   * @param {string} text
+   * @return {boolean}
+   */
+  setText(text) {
+    if (text === this._text)
+      return false;
+    this._hasUpdatedText = true;
+    this._text = text;
+    return true;
+  }
+
+  /**
+   * @return {string}
+   */
+  text() {
+    return this._text;
+  }
+
+  /**
+   * @return {boolean}
+   */
+  hasUpdatedText() {
+    return this._hasUpdatedText;
+  }
+}
+
+class SourceFactory {
+  constructor() {
+    this._sources = new Map();
+  }
+
+  /**
+   * @return {!Array<!Source>}
+   */
+  sources() {
+    return Array.from(this._sources.values());
+  }
+
+  async saveChangedSources() {
+    const changedSources = Array.from(this._sources.values()).filter(source => source.hasUpdatedText());
+    await Promise.all(changedSources.map(source => writeFileAsync(source.filePath(), source.text())));
+  }
+
+  /**
+   * @param {string} filePath
+   * @return {!Promise<Source>}
+   */
+  async readFile(filePath) {
+    filePath = path.resolve(filePath);
+    let source = this._sources.get(filePath);
+    if (!source) {
+      const text = await readFileAsync(filePath, {encoding: 'utf8'});
+      source = new Source(filePath, text);
+      this._sources.set(filePath, source);
+    }
+    return source;
+  }
+
+  /**
+   * @param {string} dirPath
+   * @param {string=} extension
+   * @return {!Promise<!Array<!Source>>}
+   */
+  async readdir(dirPath, extension = '') {
+    const fileNames = await readdirAsync(dirPath);
+    const filePaths = fileNames.filter(fileName => fileName.endsWith(extension)).map(fileName => path.join(dirPath, fileName));
+    return Promise.all(filePaths.map(filePath => this.readFile(filePath)));
+  }
+}
+
+module.exports = SourceFactory;