Safer build: copy src/node to build/node.

Add traceur-build that runs out of build/ and thus uses build/node/*.js.
Copy current src/node to build/node-save, restore last-good src/node,
copy to build/node, restore src/node. This reduces ways that src/node
changes can be lost by build shenanigans.

TBR=johnjbarton

Review URL: https://github.com/google/traceur-compiler/pull/607

Closes #607.