Build artifacts should not be produced next to source files

[mit-mit]

We currently place the build artifacts (.bin, .elf, etc.) next to the source files. We should put them inside an build directory.

[danrubel]

In Atom Dartino plugin, I can have the build artifacts placed in a build directory relative to the project directory ( e.g. /build/*/.elf ). When compiling from the command line, there is no concept of a project directory, so in which "build directory" should the build artifacts be placed?

[mit-mit]

For the command line can we just say that the build directory is a placed "next to" the .dart file passed to the compiler?

[danrubel]

Sgtm