This ensures that accidental usages of dart:io's
file API don't creep in over time.

This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.

This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.

With this change, they're run via instance methods on an object
obtained through the context. This will allow us to substitute
that object in tests with replay/record versions to allow us to
mock out the os-layer in tests.

* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8
* throw exception if exit code is not an integer
* rework command infrastructure to throw ToolExit when non-zero exitCode
* convert commands to return Future<Null>
* cleanup remaining commands to use throwToolExit for non-zero exit code
* remove isUnusual exception message
* add type annotations for updated args package

* make showElapsedTime default to true

* support nested progresses

* improve hot reload message

* rethrow

The way this was implemented is sketchy, especially in strong mode.

* refactor _run to runCmd

* replace requiresProjectRoot getter with call to commandValidator

* replace requiresDevice getter with call to findTargetDevice

* trace command requires a debug connection, not a device

* inline androidOnly getter

* rename command methods to verifyTheRunCmd and runCmd

* move common verification into BuildSubCommand

* rename deviceForCommand to device

* rename methods to verifyThenRunCommand and runCommand

Previously these widget had zero test coverage. Also, improve the
coverage tooling Mac.

`flutter test` now has a `--merge-coverage` flag that can be used to merge
coverage data from previous runs, enabling faster iteration cycles.

We need https://github.com/dart-lang/coverage/issues/100 to be fixed before
this will be useful.

Fixes #2342

This prevents multiple simultaneous runs of the analyzer from stomping
over each other (e.g. multiple runs of 'update-packages'). Certain
long-lived commands (like analyze, run, logs) are exempted once they've
done enough work to be safe from most stomping action.

This still doesn't make us entirely safe from craziness, e.g. if you're
half way through an 'update-packages' run and you call 'git pull', who
knows what state you'll end up in. But there's only so much one can do.

Fixes https://github.com/flutter/flutter/issues/2762

Also, make it clear the screen between results so it's more obvious
what's going on when you have new results (especially when you have
fixed everything).

* refactor the --resident run option into a separate file

* update daemon to run --resident apps

* re-plumbing daemon start

* send app logs

* update tests

* review changes

* fix test runner

* remove PackageMap.createGlobalInstance; rely on the ctor

* review comments

@chinmaygarde

Also, make sure that broken tests actually break the bots.

And add a test to make sure that keeps happening.

Artifacts from local engine builds will be found based on the --local-engine flag

* run pub for analyze and test

* don't run pub when doing repo analysis

* move logic for when to run pub into the commands

* re-write ternary expression

The .packages file is replacing the packages directory of symlinks.

Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

This patch switches us from using our previous hacking approach to integrating
with package:test to using the new PlatformPlugin interface.

rename to printTrace(), printStatus(), and printError()

Now you can run tests with and without --flutter-repo with a prebuilt binary on
Linux.

Fixes #307

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.

This patch still requires a locally built engine. I'll remove the need for a
locally built engine in a future patch.

Fixes #278

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.