SI-6923 Context now buffers warnings as well as errors

Code that was silently typed would not report warnings, even if it
returned a successful result.

This appeared in the following code which didn't show warnings even
with -Ywarn-adapted-args:

    def foo(a: Any) = a; foo(1, 2)

While the following would show the expected warning:

    def foo[A](a: Any) = a; foo(1, 2)
SI-6928, VerifyError with self reference to super.

A bug in typers mishandled varargs. We should get more
aggressive about eliminating all the ad hoc parameter/argument
handling code spread everywhere. For varargs especially:
any code which tries to make an adjustment based on a
repeated parameter is more likely to be wrong than right.
In aggregate these reinventions are a huge source of bugs.
SI-6912 Avoid a typer cycle in overload resolution.

c800d1fe, and followup commits 1ddc9358 and b10b5821
modified error handling in `Infer#inferExprAlternative`.
After these changes, this method could fail to resolve
the overloaded alternative if:

     best != NoSymbol
  && !competing.isEmpty
  && !noAlternatives
  && pt.isErroneous

This commit calls `setError` in that case, which
prevents the cycle in `adapt`.

While I didn't extract a reproduction from the
original code base, I've included a test case that
exhibits the same symptom.

It was actually pretty tough to find an program that
got close to this code path, but luckilly we've been
pretty close to this bug in SI-5553 / 4f99c2e5, and those
test cases formed the basis for this one.
SI-6930 adds documentation to reduceLeft in TraversableOnce
SI-6641 Deprecate SwingWorker

See discussion: https://github.com/scala/scala/pull/1862
Fixes SI-6521, overrides Range#head to be faster

Backport of 63ba3d64a7002ef67f7f13083a18fe1042a3adba
Backport of SI-6846.

Squashed commit of the following:

commit 55806cc0e6177820c12a35a18b4f2a12dc07bb39
Author: Paul Phillips <paulp@improving.org>
Date:   Wed Dec 19 07:32:19 2012 -0800

    SI-6846, regression in type constructor inference.

    In 658ba1b4e6 some inference was gained and some was lost.
    In this commit we regain what was lost and gain even more.
    Dealiasing and widening should be fully handled now, as
    illustrated by the test case.
    (cherry picked from commit dbebcd509e4013ce02655a2687b27d0967b3650e)

commit e6ef58447d0f4ef6de956fcc03ee283bb9028c02
Author: Paul Phillips <paulp@improving.org>
Date:   Fri Dec 21 15:11:29 2012 -0800

    Cleaning up type alias usage.

    I determined that many if not most of the calls to .normalize
    have no intent beyond dealiasing the type. In light of this I
    went call site to call site knocking on doors and asking why
    exactly they were calling any of

      .normalize
      .widen.normalize
      .normalize.widen

    and if I didn't like their answers they found themselves
    introduced to 'dropAliasesAndSingleTypes', the recursive widener
    and dealiaser which I concluded is necessary after all.

    Discovered that the object called 'deAlias' actually depends
    upon calling 'normalize', not 'dealias'. Decided this was
    sufficient cause to rename it to 'normalizeAliases'.

    Created dealiasWiden and dealiasWidenChain.

    Dropped dropAliasesAndSingleTypes in favor of methods
    on Type alongside dealias and widen (Type#dealiasWiden).
    These should reduce the number of "hey, the type alias doesn't work" bugs.

    (cherry picked from commit 3bf51189f979eb0dd41744ca844fd12dfdaa0dee)

    Conflicts:
    	src/compiler/scala/tools/nsc/interpreter/CompletionOutput.scala

commit c1d8803cea1523f458730103386d8e14324a9446
Author: Paul Phillips <paulp@improving.org>
Date:   Sat Dec 22 08:13:48 2012 -0800

    Shored up a hidden dealiasing dependency.

    Like the comment says:

    // This way typedNew always returns a dealiased type. This
    // used to happen by accident for instantiations without type
    // arguments due to ad hoc code in typedTypeConstructor, and
    // annotations depended on it (to the extent that they worked,
    // which they did not when given a parameterized type alias
    // which dealiased to an annotation.) typedTypeConstructor
    // dealiases nothing now, but it makes sense for a "new" to
    // always be given a dealiased type.

    PS:
    Simply running the test suite is becoming more difficult all
    the time. Running "ant test" includes time consuming activities
    of niche interest such as all the osgi tests, but test.suite
    manages to miss the continuations tests.
    (cherry picked from commit 422f461578ae0547181afe6d2c0c52ea1071d37b)

commit da4748502792b260161baa10939554564c488051
Author: Paul Phillips <paulp@improving.org>
Date:   Fri Dec 21 12:39:02 2012 -0800

    Fix and simplify typedTypeConstructor.

    Investigating the useful output of devWarning (-Xdev people,
    it's good for you) led back to this comment:

      "normalize to get rid of type aliases"

    You may know that this is not all the normalizing does.
    Normalizing also turns TypeRefs with unapplied arguments
    (type constructors) into PolyTypes. That means that when
    typedParentType would call typedTypeConstructor it would
    find its parent had morphed into a PolyType. Not that it
    noticed; it would blithely continue and unwittingly discard
    the type arguments by way of appliedType (which smoothly
    logged the incident, thank you appliedType.)

    The simplification of typedTypeConstructor:

    There was a whole complicated special treatment of AnyRef
    here which appears to have become unnecessary. Removed special
    treatment and lit a candle for regularity.

    Updated lots of tests regarding newly not-so-special AnyRef.
    (cherry picked from commit 394cc426c1ff1da53146679b4e2995ece52a133e)

commit 1f3c77bacb2fbb3ba9e4ad0a8a733e0f9263b234
Author: Paul Phillips <paulp@improving.org>
Date:   Fri Dec 21 15:06:10 2012 -0800

    Removed dead implementation.

    Another "attractive nuisance" burning off time until I
    realized it was commented out.
    (cherry picked from commit ed40f5cbdf35d09b02898e9c0950b9bd34c1f858)
SI-6555 Scaladoc's class filter shouldn't drop the last character

The event handler have to wait "keyup", not "keydown".
SI-6946, SI-6924 Greatly improves IsTraversableLike docs
SI-5189 detect unsoundness when inferring type of match

GADT skolems encode type slack that results from pattern matching on variant type constructors
I thought they would not longer be relevant after cases have been typed,
and since they caused weird issues with the old pattern matcher, I deskolemized in typedCase

however, when we don't have an expected type for the match, we need to keep the skolems around
until the skolemized type makes it out of the match and it becomes the result of type inference for that match
when you do have an expected type, it will propagate to the case-level and the confrontation will thus
already take place when typing individual cases
SI-6925 use concrete type in applyOrElse's match's selector

Fix a regression introduced in 28483739c3:

PartialFunction synthesis was broken so that we'd get:
```
scala> def f[T](xs: Set[T]) = xs collect { case x => x }
f: [T](xs: Set[T])scala.collection.immutable.Set[_ <: T]
```

rather than
```
scala> def f[T](xs: Set[T]) = xs collect { case x => x }
f: [T](xs: Set[T])scala.collection.immutable.Set[T]
```
rework partial function synthesis

no behavioral changes, just highly overdue cleanup
some TODOs for further improvements
clean up synthesizePartialFunction

implement the following review comments by @retronym:
- [x] Please clothe this naked assert.
- [x] Use match to dissect targs and check isFullyDefined.
- [x] Instead of `targs.head`/`targs.last`, use `val argTp :: resTp :: Nil = targs`.
- [x] Add a quasi-quote-style comment for `apply`.
- [x] Factor out mkCastPreservingAnnotations.
SI-6955 switch emission no longer foiled by type alias

dealias the type of the scrutinee before checking it's switchable
now with tests! (using IcodeTest since javap is not available everywhere)
Merge pull request #1863 from retronym/ticket/6641-2.10.x

SI-6641 Deprecate SwingWorker
Merge pull request #1853 from paulp/issue/6928

SI-6928, VerifyError with self reference to super.
Merge pull request #1869 from retronym/backport/1826

Backport of SI-6846.
Merge pull request #1859 from retronym/ticket/6912

SI-6912 Avoid a typer cycle in overload resolution.
SI-5340 Change println to log

An esoteric implicit search could trigger an "amb prefix ..."
message to standard out. Now the message has been improved
and sent to the logger.
SI-5440 Test case for exhaustiveness check

Reported against patmatclassic, working in virtpatmat.