Add a 'flutter ios --init' command that fetches the Xcode project from the cloud and configures it for a given flutter project

1) Moved basic utility code into base/ directory to make it clear which code
   doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
   aren't commands themselves.

Example:
   cd flutter/examples/stocks
   flutter --engine-src-path /path/to/engine/src apk -o Stocks.apk -m apk/AndroidManifest.xml

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

wrap up first daemon implementation

remove an unused import

review comments

rename st --> stack

Instead, just print the usage.

This patch makes `flutter start` work without a clone of the engine git
repository. Making this work pulled a relatively large refactor of how the
commands interact with application packages and devices. Now commands that want
to interact with application packages or devices inherit from a common base
class that holds stores of those objects as members.

In production, the commands download and connect to devices based on the build
configuration stored on the FlutterCommandRunner. In testing, these fields are
used to mock out the real application package and devices.

Expose the main entry point for the tools via the library lets us run the tools
from the Flutter package, which simplifies the setup for end developers because
they don't need to declare a dependency on sky_tools directly.

I'm trying to get a feel for the code by writing some simple cleanup patches.

Add listen command and basic test, and don’t do unnecessary repeated work when listening or poking the android server.

This makes the 'package-root' option universal for sky_tools and configures the
ArtifactStore with it statically at startup. The actual sky_engine revision
is computed on demand.

Refactor all the commands to be Commands from the Args package.  Also use CommandRunner for the top-level command.

This adds the following commands to sky_tools:
  sky_tools cache clear: Nukes all local artifacts in the cache
  sky_tools cache populate: Populates the cache with all known artifacts

This is useful both to fix busted caches and to make sure that the cache is
fully populated so that subsequent operations can proceed without needing
network access.

`dart bin/sky_tools.dart --debug --sky-src-path=/path/to/sky/src/ install` now works.

This initial version assumes the developer has mojo_shell and all other services
sitting on disk somewhere and that they're on linux and only want to run on
linux. This can be generalized down the line to support more use cases. This
downloads the sky_viewer.mojo corresponding to the packages/sky_engine/REVISION
in the developer's directory, so they can specify whatever revision they want.
sky_tools run_mojo downloads sky_viewer.mojo into its cache directory if it is
not present and constructs a command line to pass to mojo_shell that maps the
shebang stamped into the flx to the downloaded sky_viewer.mojo.

Since sky_viewer.mojo lives in the cloud and mojo_shell can load from the cloud
this could also map to an https URL. This should likely be an option.

This command will produce an flx package. Currently, this command doesn't work
because we don't have the Flutter compiler downloaded from Google storage yet.
A future patch will make that happen.

Add very-verbose flag and automatically log some more process-related things in verbose and very-verbose modes.