Return remote object description for unserializable objects

This patch starts handling unserializable objects by returning their
description.

Fixes #86.

lib/helper.js

@@ -75,13 +75,21 @@ class Helper {
     }
     if (!remoteObject.objectId)
       return remoteObject.value;
-    let response = await client.send('Runtime.callFunctionOn', {
-      objectId: remoteObject.objectId,
-      functionDeclaration: 'function() { return this; }',
-      returnByValue: true,
-    });
-    client.send('Runtime.releaseObject', {objectId: remoteObject.objectId});
-    return response.result.value;
+    try {
+      let response = await client.send('Runtime.callFunctionOn', {
+        objectId: remoteObject.objectId,
+        functionDeclaration: 'function() { return this; }',
+        returnByValue: true,
+      });
+      return response.result.value;
+    } catch (e) {
+      // Return description for unserializable object, e.g. 'window'.
+      return remoteObject.description;
+    } finally {
+      client.send('Runtime.releaseObject', {objectId: remoteObject.objectId}).catch(e => {
+        // While we were serializing object, the page might've navigated.
+      });
+    }
   }
 }
 

test/test.js

@@ -128,6 +128,10 @@ describe('Puppeteer', function() {
       let result = await page.evaluate(() => -Infinity);
       expect(Object.is(result, -Infinity)).toBe(true);
     }));
+    it('should not fail for window object', SX(async function() {
+      let result = await page.evaluate(() => window);
+      expect(result).toBe('Window');
+    }));
   });
 
   describe('Page.injectFile', function() {
@@ -256,6 +260,13 @@ describe('Puppeteer', function() {
         'calling console.error',
       ]);
     }));
+    it('should not fail for window object', SX(async function() {
+      let windowObj = null;
+      page.once('console', arg => windowObj = arg);
+      page.evaluate(() => console.error(window));
+      await waitForEvents(page, 'console');
+      expect(windowObj).toBe('Window');
+    }));
   });
 
   describe('Page.navigate', function() {