Rename file System.js to LoaderHooks.js

Review URL: https://github.com/google/traceur-compiler/pull/596

Closes #596.
updates link to REPL

Review URL: https://github.com/google/traceur-compiler/pull/597

Review URL: https://github.com/google/traceur-compiler/pull/599

Closes #599, closes #597.
Fix node-front-end build rules and build/dep.mk

Build intermediate traceur.js with last-good node front.
./traceur combines bin/traceur.js with src/node code.
While self-compiling, we use last-known good bin/traceur.js
from git and now we use last-known good src/node.

Emit the build/dep.mk makefile rules in evalCodeUnit() step.
Don't automatically make build/dep.mk, its dependencies are too
complex.

Review URL: https://github.com/google/traceur-compiler/pull/600

Closes #600.