Without this, an FLX with no assets may be completely empty in AOT mode.
This will result in a warning when the engine's unzip library tries to
parse the FLX.
Fixes https://github.com/flutter/flutter/issues/7060
This change adds a top-level getBuildDirectory func and funcs for
android, aot, asset, ios build products.
Developers may now add a "build-dir" mapping to their
~/.flutter_settings (JSON format) config file. Output directory is
relative to the main flutter application directory.
This change also changes the default build directory for iOS builds to a
subdirectory of the configured build directory, 'build/ios' by default.
