Build traceur-runtime.js with package name traceur-runtime.

Fixes #701, Polyfills not included in bin/traceur-runtime.js.
Distinct package name ensures that, while compiling the runtime, we won't mix
input files with traceur modules precompiled in the compiler.
Add test for bin/traceur-runtime.js including polyfill module.
Fix up System object initialization to allow browser test/runtime.html to run.

Review URL: https://github.com/google/traceur-compiler/pull/706

Closes #706, closes #701.