Add documentation to parts of scala.concurrent.
Don't crash on dead code in ICodeReader
Tone down the noise in the output of t7974

This makes it pass under -Ybackend:GenBCode
Make test repl-javap-app independent of precise constant pool indices

Allows the test to pass on both backends
Test: classfile reading during inlining works if there is dead code
Merge pull request #3816 from lrytz/bcode/bugs

Don't crash on dead code in ICodeReader
Merge pull request #3787 from tvierling/concurrent-doc-nits

Add documentation to parts of scala.concurrent.
SI-5919 TypeTags and Exprs should be serializable

- Make TypeCreator and TreeCreator extend Serializable.
- When replacing a SerializedTypeTag with a TypeTag or WeakTypeTag,
  do not use scala.reflect.runtime.universe.rootMirror, since
  it is unlikely to find user classes; instead, create a runtime
  mirror using the context ClassLoader of the current thread.
  Use the same logic for SerializedExpr.
- Remove writeObject/readObject methods from SerializedTypeTag
  and SerializedExpr since they are unused.
- Add @throws annotation on writeReplace and readResolve methods.
- Handle SecurityException if the current thread cannot access the
  context ClassLoader.
- To make type tags of primitive value classes serializable, make
  PredefTypeCreator a top-level class. Otherwise, it would
  retain a reference to the enclosing Universe,
  rendering the TypeCreator non-serializable.

Binary compatibility:

- Keep nested PredefTypeCreator class to avoid binary incompatible
  change.
- Keep `var` modifiers on the class parameters of
  SerializedTypeTag.