Implement Frame.evaluate

This patch implements Frame.evaluate method.

References #4.

lib/helper.js

@@ -0,0 +1,57 @@
+/**
+ * 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.
+ */
+
+class Helper {
+  /**
+   * @param {function()} fun
+   * @param {!Array<*>} args
+   * @return {string}
+   */
+  static evaluationString(fun, ...args) {
+    return `(${fun})(${args.map(x => JSON.stringify(x)).join(',')})`;
+  }
+
+  /**
+   * @param {!Connection} client
+   * @param {!Object} exceptionDetails
+   * @return {string}
+   */
+  static async getExceptionMessage(client, exceptionDetails) {
+    let message = '';
+    let exception = exceptionDetails.exception;
+    if (exception) {
+      let response = await client.send('Runtime.callFunctionOn', {
+        objectId: exception.objectId,
+        functionDeclaration: 'function() { return this.message; }',
+        returnByValue: true,
+      });
+      message = response.result.value;
+    } else {
+      message = exceptionDetails.text;
+    }
+
+    if (exceptionDetails.stackTrace) {
+      for (let callframe of exceptionDetails.stackTrace.callFrames) {
+        let location = callframe.url + ':' + callframe.lineNumber + ':' + callframe.columnNumber;
+        let functionName = callframe.functionName || '<anonymous>';
+        message += `\n    at ${functionName} (${location})`;
+      }
+    }
+    return message;
+  }
+}
+
+module.exports = Helper;