add a bot to test Flutter

[devoncarew]

Some work towards #1317:

• add a bot to test Flutter (we now have the main bot, the dart sdk generation bot, and a flutter bot)
• also, echo bash commands as we execute them, and some minor tweaks and cleanup to the bash script

@jcollins-g

[devoncarew]

(bot results visible here: https://travis-ci.org/dart-lang/dartdoc/builds/223015439)

[devoncarew]

I do see an exception on generation here: https://travis-ci.org/dart-lang/dartdoc/jobs/223015441#L444

Generation failed: Stack Overflow
package:analyzer/src/dart/resolver/inheritance_manager.dart 266:61  InheritanceManager._computeClassChainLookupMap
package:analyzer/src/dart/resolver/inheritance_manager.dart 286:15  InheritanceManager._computeClassChainLookupMap
package:analyzer/src/dart/resolver/inheritance_manager.dart 123:7   InheritanceManager.getMembersInheritedFromClasses
package:dartdoc/src/model.dart 421:36                               Class.inheritedMethods
package:dartdoc/src/model.dart 277:18                               Class.allInstanceMethods
package:dartdoc/src/model.dart 1458:26                              Library.modelElementsMap.<fn>
package:dartdoc/src/model.dart                                      Library.modelElementsMap
package:dartdoc/src/model.dart 2802:19                              Package.findCanonicalLibraryFor
package:dartdoc/src/model.dart 2793:12                              Package.isDocumented
package:dartdoc/src/model.dart 525:20                               Class.inheritedOperators
package:dartdoc/src/model.dart 310:18                               Class.allOperators
package:dartdoc/src/model.dart 1460:26                              Library.modelElementsMap.<fn>
package:dartdoc/src/model.dart                                      Library.modelElementsMap
package:dartdoc/src/model.dart 2802:19                              Package.findCanonicalLibraryFor
.                                                                   ...
===== asynchronous gap ===========================
dart:async                                                          _Completer.completeError
package:dartdoc/dartdoc.dart 184:3                                  DartDoc.generateDocs
===== asynchronous gap ===========================
dart:async                                                          Future.Future.microtask
package:dartdoc/dartdoc.dart 116:47                                 DartDoc.generateDocs
../../../../bin/dartdoc.dart 173:44                                 main.<fn>
===== asynchronous gap ===========================
dart:async                                                          Future.Future.microtask
../../../../bin/dartdoc.dart 172:26                                 main.<fn>
package:stack_trace                                                 Chain.capture
../../../../bin/dartdoc.dart 172:9                                  main

[jcollins-g]

We should never get into a findCanonicalLibrary loop; this is a dartdoc problem, though I'm surprised I haven't seen it before. Looking at your bot setup script.

[jcollins-g]

The output from travis looks wrong; for me, flutter has exactly one ambiguous reexport (Matrix4). So something about how this is being launched seems to be creating a harder problem for dartdoc to solve.

[devoncarew]

I'll take another look at this as see if I'm doing anything strange in the setup.

[devoncarew]

@jcollins-g, I took another look at the script; looks reasonable to me. Appreciate any feedback on it, and when you think we should land this PR - while we still have issues w/ flutter generation or after.

[devoncarew]

(new build in progress: https://travis-ci.org/dart-lang/dartdoc/builds/223610320)