Merge 2.10.x into master

Here's the merge plan I used:

```
$ g co 2.10.x && g pull scala 2.10.x && g co master && g pull scala master
$ export MB=`git merge-base 2.10.x master` # 9cdbe28
$ git log --graph --oneline $MB...2.10.x

*   b3ad753 (scala/2.10.x, 2.10.x) Merge pull request scala#3364 from retronym/ticket/8152
|\
| * 9df2dcc (scala/pr/3364) [nomaster] SI-8152 Backport variance validator performance fix
* |   d5801b9 Merge pull request scala#3328 from retronym/ticket/8111
|\ \
| |/
|/|
| * c91d373 (scala/pr/3328) SI-8111 Expand the comment with a more detailed TODO
| * 2c770ae (scala/pr/3327) SI-8111 Repair symbol owners after abandoned named-/default-args
* |   a812241 Merge pull request scala#3345 from retronym/ticket/8114
|\ \
| * | 5876e8c (scala/pr/3345) [nomaster] SI-8114 Binary compat. workaround for erasure bug SI-7120
* | |   aefe3fb Merge pull request scala#3344 from jamesward/fix/ec-implicit-error
|\ \ \
| |/ /
|/| |
| * | bd4adf5 (scala/pr/3344) More clear implicitNotFound error for ExecutionContext
|/ /
* |   1f2cd7e (origin/2.10.x) Merge pull request scala#3329 from retronym/ticket/6563
|\ \
| |/
|/|
| * 255c51b (scala/pr/3329) SI-6563 Test case for already-fixed crasher
|/
*   370d6d6 Merge pull request scala#3312 from xeno-by/topic/fine-points-of-whiteboxity-210x
|\
| * c0cb1d8 (scala/pr/3312) [nomaster] codifies the state of the art wrt SI-8104
|/
*   97b9b2c Merge pull request scala#3282 from retronym/ticket/8085
|\
| * 7e85b59 (scala/pr/3282) SI-8085 Fix BrowserTraverser for package objects
| * a12dd9c Test demonstrating SI-8085
|/
*   5cbb5a7 (tag: v2.10.4-RC1) Merge pull request scala#3268 from adriaanm/support-3021
|\
| * 3fa2c97 (scala/pr/3268, adriaanm/support-3021) Report error on code size overflow, log method name.
|/
* 2aa9da5 Partially revert f8d8f7d.
*   ba7bf5e Merge pull request scala#3261 from adriaanm/ticket-6426
|\
| * 47562e7 (scala/pr/3261, adriaanm/ticket-6426, ticket-6426) Revert "SI-6426, importable _."
* da919ec Merge pull request scala#3253 from retronym/ticket/8062
* f0d913b (scala/pr/3253) SI-8062 Fix inliner cycle with recursion, separate compilation

$ g log --oneline --no-merges $MB..97b9b2c
7e85b59 (scala/pr/3282) SI-8085 Fix BrowserTraverser for package objects
a12dd9c Test demonstrating SI-8085
3fa2c97 (scala/pr/3268, adriaanm/support-3021) Report error on code size overflow, log method name.
2aa9da5 Partially revert f8d8f7d.
47562e7 (scala/pr/3261, adriaanm/ticket-6426, ticket-6426) Revert "SI-6426, importable _."
f0d913b (scala/pr/3253) SI-8062 Fix inliner cycle with recursion, separate compilation

$ g log --oneline --no-merges 97b9b2c..370d6d6
c0cb1d8 (scala/pr/3312) [nomaster] codifies the state of the art wrt SI-8104

$ g log --oneline --no-merges 370d6d6..aefe3fb
bd4adf5 (scala/pr/3344) More clear implicitNotFound error for ExecutionContext
255c51b (scala/pr/3329) SI-6563 Test case for already-fixed crasher

$ g log --oneline --no-merges aefe3fb..a812241
5876e8c (scala/pr/3345) [nomaster] SI-8114 Binary compat. workaround for erasure bug SI-7120

$ g log --oneline --no-merges a812241..d5801b9
c91d373 (scala/pr/3328) SI-8111 Expand the comment with a more detailed TODO
2c770ae (scala/pr/3327) SI-8111 Repair symbol owners after abandoned named-/default-args

$ g log --oneline --no-merges d5801b9..2.10.x
9df2dcc (scala/pr/3364) [nomaster] SI-8152 Backport variance validator performance fix

$ g merge 97b9b2c
$ g merge -s ours 370d6d6
$ g merge aefe3fb
$ g merge -s ours a812241
$ g merge d5801b9
$ g merge -s ours 2.10.x

```

Author: adriaanm