Load runtime.js only once

runtime.js is sometimes inlined into other modules such as es6-module-transpiler and results in "Invalid access to private name"[1] when used with such modules.

Making runtime.js prevents from being executed multiple times solves the problem basically.

[1] https://github.com/thlorenz/es6ify/issues/3

Review URL: https://github.com/google/traceur-compiler/issues/323

Closes #323.