CI: ensure that example code is analyzed and tested

[chalin]

Reinstate code checks that were temporarily disabled by #2738:

• Previous working build: https://travis-ci.org/flutter/website/builds/539244102
• First broken build: https://travis-ci.org/flutter/website/builds/539735108 -- not that this build also failed because of a broken link (404.
• First working build: https://travis-ci.org/flutter/website/builds/542865401

Related:

• Code excerpts: use new tooling and infra Code excerpts: use new tooling and infra #2233

[chalin]

The problem is that Travis Ubuntu uses GNU diff 3.x and macOS is still at 2.x (the macOS version is very old).