Replace all instances of console.assert() with assert() from 'assert' standard lib.

[usergenic]

As of node 10, console.assert() doesn't throw an AssertionError anymore. However const assert = require('assert'); assert(); does. So I switched over to assert() wherever console.assert() was in use.

[rictic]

Looks like appveyor is running with node 10 and no longer failing where it was before. Now we've just got a minor whitespace difference in the polymer build integration test goldens. Mind fixing that too in this PR?

[rictic]

Hm, weirdly though, node 6 passes on travis without whitespace changes…

[usergenic]

Side note: I believe Node 10 has issues (even locally) in polymer-build tests with some dependency of the temp library we use to make directories for test scenarios:

     Uncaught RangeError [ERR_OUT_OF_RANGE]: The value of "mode" is out of range. Received 16877
      at Object.fs.fchmod (fs.js:1024:11)
      at Object.fchmod (node_modules/graceful-fs/polyfills.js:237:17)
      at mode (node_modules/vinyl-fs/lib/fileOperations.js:132:10)
      at onStat (node_modules/vinyl-fs/lib/fileOperations.js:125:14)
      at node_modules/graceful-fs/polyfills.js:287:18
      at FSReqWrap.oncomplete (fs.js:150:5)

I get this locally and on travis for node 10: