Fix #275: Remove isScalaJSDefined (and company)

The Scala.js compiler relies on the ScalaJS library being on the
classpath since the introduction of RuntimeLong. Therefore, checking
for isScalaJSDefined can only achieve the following two:

- Increase clutter
- Give the wrong impression that the ScalaJS library need not be on
  the classpath.

This commit removes isScalaJSDefined and references to it.
Merge pull request #277 from gzm0/remove-isjsdefined

Fix #275: Remove isScalaJSDefined (and company)
Fix #278: Remove dead-code: truncateToLong
Fix #280: Inline dynamicIsInstanceOf
Fix #279: Simplify and inline num2char
Merge pull request #282 from gzm0/code-cleanup

Code cleanup
Fix #284: Remove var currentCUnit in GenJSCode
Fix #287: Remove reflective call proxies for default param methods
Fix #285: Make @JSName work for getters and setters
Merge pull request #289 from gzm0/minor-fixes

Minor fixes
Comment behavior/rationale of check-partest-coverage.sh
Merge pull request #291 from gzm0/comment-partest-coverage

Comment behavior/rationale of check-partest-coverage.sh
Fix #286: Disallow non-unit return setters in facade types
Merge pull request #293 from gzm0/setter-check

Fix #286: Disallow non-unit return setters in facade types
Do not emit code for impl classes of raw JS traits.
Merge pull request #294 from sjrd/raw-js-impl-classes

Do not emit code for impl classes of raw JS traits.