Add an example for launching a URL

Bump dependencies on sky_engine and sky_services

Cursor is tiny when there's no text in an input

Now we use a zero-width space to force the engine to resolve the font and tell
us how large the text is likely to be once there's text in the widget.

Fixes #302

Fix tapcancel on slop.

Hit Testing Cleanup

More detailed debugging info for RenderPointerListener et al

Trivial whitespace fixes.

Make sure to send tapcancel when the primary pointer fails because of
slop, even if the gesture won by default.

Also, minor cleanup and clarification of an invariant.

Elaborate on the debugging information exposed by
RenderPointerListener, Listener, and GestureDetector.

- Drop the unused BindingHitTestEntry class. (Well, it was
  constructed, but its member was never used, so it wasn't doing
  anything a boring old HitTestEntry couldn't already do.)

- Add toString()s to HitTestEntry and HitTestResult, to aid in
  debugging hit-test related code.

Explicitly dispose heavy C++ objects

This patch calls dispose() explicitly on a number of heavy C++ objects so that
they can be eagerly finalized instead of waiting for GC.

Parent data not updated when reparenting using global keys

When we reactivate a subtree that had a global key, we weren't updating the
parent data because:

1) The child wasn't in the tree when we updated the parent data element.
2) The activated child didn't go through mount (just through update).

This patch moves the parent data update work to when we attach the render
object, which we do both during mount and when reactivating a child.

Fixes #345

Fix Dialog and PopupMenu completion logic

It's now (just) in TransitionRoute.

Added smoke tests for showMenu() and showDialog().

Fixes #337

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Stable is way too out of date, e.g. it still complains about libraries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.