Merge 2.11.x to 2.12.x

One conflict in build.xml around the addition of the property
`test.bc.skip`. Straight forward to resolve.

```
% export MB=$(git merge-base origin/2.12.x origin/2.11.x)

% git --no-pager log --oneline --graph --decorate $MB..origin/2.11.x
*   223e207 (origin/2.11.x, 2.11.x) Merge pull request #3848 from Ichoran/issue/8680
|\
| * 039f3e3 (origin/pr/3848) SI-8680  Stream.addString is too eager
*   75f1235 Merge pull request #3941 from Ichoran/issue/8815
|\
| * 1da69e8 (origin/pr/3941) SI-8815  mutable.LongMap makes different choices for splitAt vs etc.
*   eb5d0a6 Merge pull request #3936 from som-snytt/issue/8806
|\
| * c9ec916 (origin/pr/3936) SI-8806 Add lower bound check to Any lint
*   3307e44 Merge pull request #3972 from lrytz/BCodeDelambdafyFix
|\
| * 63207e1 (origin/pr/3972) isAnonymousClass/Function for delambdafy classes is not true
* 0e2be38 Merge pull request #3971 from lrytz/opt/dce
* 9132efa (origin/pr/3971) Address review feedback.
* 59070cc Remove stale local variables and exception handlers after DCE
* 01f5435 SI-8568 unreachable test now passes in GenBCode
* 630bd29 Clarify why we emit ATHROW after expressions of type Nothing
* 35c53af Tools to run the compiler in JUnit tests
* 44b5c26 JUnit tests for dead code elimination.
* d3cfbb1 -Yopt mulit-choice flag
* 90781e8 Eliminate unreachable code in GenBCode
* a3dfb43 Removed empty class, unused imports

% git merge origin/2.11.x
Removing test/files/jvm/unreachable.flags
Auto-merging src/library/scala/collection/mutable/LongMap.scala
Auto-merging build.xml
CONFLICT (content): Merge conflict in build.xml
Automatic merge failed; fix conflicts and then commit the result.
```