page.evaluate takes a string in addition to function (#135)

This patch improves on page.evaluate to accept a string.
The string can have a trailing '//# sourceURL=' comment which would
name the evaluation to make stacks beautiful.

In order to make sourceURL comments possible, this patch:
- removes wrapping of the client function into `Promise.resolve()`
- stops passing `awaitPromise` parameter to `Runtime.evaluate`
- starts to await promise via the `Runtime.awaitPromise` if the return type of the evaluation
  is promise

closes #118

lib/helper.js

@@ -16,11 +16,15 @@
 
 class Helper {
   /**
-   * @param {function()} fun
+   * @param {function()|string} fun
    * @param {!Array<*>} args
    * @return {string}
    */
   static evaluationString(fun, ...args) {
+    if (typeof fun === 'string') {
+      console.assert(args.length === 0, 'Cannot evaluate a string with arguments');
+      return fun;
+    }
     return `(${fun})(${args.map(x => JSON.stringify(x)).join(',')})`;
   }
 
@@ -48,6 +52,16 @@ class Helper {
    * @return {!Promise<!Object>}
    */
   static async serializeRemoteObject(client, remoteObject) {
+    if (remoteObject.subtype === 'promise') {
+      let response = (await client.send('Runtime.awaitPromise', {
+        promiseObjectId: remoteObject.objectId,
+        returnByValue: false
+      }));
+      Helper.releaseObject(client, remoteObject);
+      if (response.exceptionDetails)
+        throw new Error('Evaluation failed: ' + Helper.getExceptionMessage(response.exceptionDetails));
+      remoteObject = response.result;
+    }
     if (remoteObject.unserializableValue) {
       switch (remoteObject.unserializableValue) {
         case '-0':