Try to simplify our bot setup. (#3986)

This moves all the bot-related files to `dev/bots`, hiding it from our
home page in github. Also, simplifies the travis setup, though that
doesn't do any difference to the performance sadly.

.travis.yml

-language: dart
 sudo: false
-dart:
-  - stable
 addons:
   apt:
+    # sky_shell binary depends on /usr/lib/x86_64-linux-gnu/libstdc++.so.6 version GLIBCXX_3.4.18
     sources:
-    - ubuntu-toolchain-r-test
+      - ubuntu-toolchain-r-test # if we don't specify this, the libstdc++6 we get is the wrong version
     packages:
-    - gcc-4.8
-    - g++-4.8
+      - libstdc++6
 before_script:
-  - ./travis/setup.sh
+  - ./dev/bots/setup.sh
 script:
-  - ./travis/test.sh
+  - ./dev/bots/test.sh
 cache:
   directories:
     - $HOME/.pub-cache

infra/README.md → dev/bots/README.md

@@ -2,14 +2,19 @@
 
 This directory exists to support building Flutter on our build infrastructure.
 
-The results of such builds are viewable at
-https://build.chromium.org/p/client.flutter/waterfall
+The results of such builds are viewable at:
+* https://build.chromium.org/p/client.flutter/waterfall
+* https://travis-ci.org/flutter/flutter/builds (limited checking used just for PRs on github)
 
-The external master pages do not allow forcing new builds. Contact
-@eseidelGoogle or another member of Google's Flutter team if you need to do
-that.
+The external master pages for the chromium infra bots do not allow
+forcing new builds. Contact @eseidelGoogle or another member of
+Google's Flutter team if you need to do that.
 
-Our infrastructure is broken into two parts.  A buildbot master specified by our
+The Travis-based bots are trivial, and just run a couple of shell
+scripts. The rest of this document discusses only the chromium infra
+bots.
+
+This infrastructure is broken into two parts. A buildbot master specified by our
 [builders.pyl](https://chromium.googlesource.com/chromium/tools/build.git/+/master/masters/master.client.flutter/builders.pyl)
 file, and a [set of
 recipes](https://chromium.googlesource.com/chromium/tools/build.git/+/master/scripts/slave/recipes/flutter)