add a flutter.yaml file for the widgets project

Make the analyzer actually work.

Turns out that ignoring all error lines that match the empty string is a
poor way to go.

Also, we have to update all the example packages now too, since we
analyze them. So just have travis use our update script.

Also, remove flutter_tools' old travis stuff. It's now part of a bigger
repo.

Also, make travis use the dev Dart SDK, since we need the new analyzer.
Stable is way too out of date, e.g. it still complains about libraries
not having names and mixins using 'super', and the strong mode hints are
even more aggressive than on dev.

Delete analyzer temporary directories

Remove constructor tear-offs

Some of our workflows have no way to silence analyzer warnings, and the
analyzer doesn't yet support constructor tear-offs, so pretend that we
can't use them for now.

flutter analyze command

ItemBuilder should also get the index of the item

Other changes in this patch:
- Make the 'flutter' tool say "Updating flutter tool..." when it calls
  pub get, to avoid confusion about what the pub get output is about.
- Make the bash flutter tool call pub get when the revision has
  changed. (This was already happening on Windows.)
- Fix a raft of bugs found by the analyzer.
- Fix some style nits in various bits of code that happened to be near
  things the analyzer noticed.
- Remove the logic in "flutter test" that would run "pub get", since
  upon further reflexion it was determined it didn't work anyway.
  We'll probably have to add better diagnostics here and say to run the
  updater script.
- Remove the native velocity tracker script, since it was testing code
  that has since been removed.

Notes on ignored warnings:
- We ignore warnings in any packages that are not in the Flutter repo or
  in the author's current directory.
- We ignore various irrelevant Strong Mode warnings. We still enable
  strong mode because even though it's not really relevant to our needs,
  it does (more or less accidentally) catch a few things that are
  helpful to us.
- We allow CONSTANTS_LIKE_THIS, since we get some of those from other
  platforms that we are copying for sanity and consistency.
- We allow one-member abstract classes since we have a number of them
  where it's perfectly reasonable.
- We unfortunately still ignore warnings in mojom.dart autogenerated
  files. We should really fix those but that's a separate patch.
- We verify the actual source file when we see the 'Name non-constant
  identifiers using lowerCamelCase.' lint, to allow one-letter variables
  that use capital letters (e.g. for physics expressions) and to allow
  multiple-underscore variable names.
- We ignore all errors on lines that contain the following magic
  incantation and a "#" character:
    // analyzer doesn't like constructor tear-offs
- For all remaining errors, if the line contains a comment of the form
    // analyzer says "..."
  ...then we ignore any errors that have that "..." string in them.

This seems useful for numbering the entries in the list, for example.

Fixes #328

Update to a newer sky_engine and sky_services

Add a `flutter upgrade` command

This command upgrades the version of flutter that you're using and runs
`pub upgrade` to update the transitive dependencies.

showSnackBar() returns a Future, clears its placeholder

The returned Future completes after the snack bar has been dismissed.

Revised BottomSheet to ensure that its Future only runs after the bottom sheet has been dismissed.

TransitionRoute completer is now optional.

also listen for flutter in the adb logs

update double tap reference

service_registry.dart should use the service registry handle instead of shell handle

Fixes assert connecting to Activity service under Linux

Update the interactive_flex example for the new event handlers

shell handle.

R=@abarth,@tonygentilcore

Fixes #313.

Add a persistent bottom sheet to the stocks demo

Move data out of Github and into Firebase

@abarth @collinjackson

Fix adb parse

Update to match Hixie's changed bash script

Adding windows batch file to complement the bash script.

Raw keyboard example