Test that flutter analyze catches no error if two imported libraries
have the same name.

Also, make tests know how to find the flutter root and fix style in one
test to be consistent with the rest of the file.

https://github.com/flutter/flutter/issues/2368

Handle this error condition better and tweak some of the formating for the
version string.

Fixes #2162

...and some style nits.

Downloading these packages from cloud storage simplifies our deployment
story because we can upload to cloud storage automatically from the
buildbot.

This patch also switches the responsibility for downloading the engine
artifacts to update_engine.sh. Centralizing this responsibility ensures
that the packages and the binaries are always in sync.

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

Before we didn't know how to find the packages of random files. Now we do.

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.