SI-8925, SI-7407 test cases, fixed in new backend

Given that the issues are old (2.10) and we move to the new backend
in 2.12, I don't plan to fix them in GenASM.

The test case for 7407 existed already, this commit just adds
`-Yopt:l:none` to the flags to make no dead code is eliminated.
SI-6502 Reenables loading jars into the running REPL (regression in 2.10)

Fixes SI-6502, reenables loading jars into the running REPL
(regression in 2.10). This PR allows adding a jar to the compile
and runtime classpaths without resetting the REPL state (crucial
for Spark SPARK-3257).

This follows the lead taken by @som-snytt in PR #3986, which
differentiates two jar-loading behaviors (muddled by cp):

  - adding jars and replaying REPL expressions (using replay)
  - adding jars without resetting the REPL (deprecated cp,
    introduced require) This PR implements require (left
    unimplemented in #3986)

This PR is a simplification of a similar approach taken by
@gkossakowski in #3884. In this attempt, we check first to make
sure that a jar is only added if it only contains new
classes/traits/objects, otherwise we emit an error. This differs
from the old invalidation approach which also tracked deleted
classpath entries.
SI-6502 Addressing review comments
SI-6502 Addresses @som-snytt's feedback
SI-6502 Addresses comments by @som-snytt
Adds tests that programatically generate jars
Addresses review comments
SI-6502 Moving methods concerned with the state of Global from IMain to Global
SI-7596 Curtail overloaded symbols during unpickling

In code like:

    object O { val x = A; def x(a: Any) = ... }
    object P extends O.x.A

The unpickler was using an overloaded symbol for `x` in the
parent type of `P`. This led to compilation failures under
separate compilation.

The code that leads to this is in `Unpicklers`:

    def fromName(name: Name) = name.toTermName match {
      case nme.ROOT     => loadingMirror.RootClass
      case nme.ROOTPKG  => loadingMirror.RootPackage
      case _            => adjust(owner.info.decl(name))
    }

This commit filters the overloaded symbol based its stability
unpickling a singleton type. That seemed a slightly safer place
than in `fromName`.
Correct collections variable definitions to avoid many scaladoc warnings.
SI-8967 Only add JARs and dirs from $SCALA_HOME/lib to classpath

That's all we ship in that directory, but it seems that some
JVMs freak out with a core dump if something else ends up
in that directory and we add it to the boot classpath.

Testing on unix:

    % rm ./build/pack/bin.complete; ant
    % touch /Users/jason/code/scala/build/pack/lib/foo.txt
    % mkdir /Users/jason/code/scala/build/pack/lib/dir
    % bash -x ./build/pack/bin/scala -version
    ...
    /Library/Java/JavaVirtualMachines/jdk1.8.0_20.jdk/Contents/Home/bin/java -Xmx256M -Xms32M -Xbootclasspath/a:/Users/jason/code/scala/build/pack/lib/dir:/Users/jason/code/scala/build/pack/lib/jline.jar:/Users/jason/code/scala/build/pack/lib/scala-actors.jar:/Users/jason/code/scala/build/pack/lib/scala-compiler.jar:/Users/jason/code/scala/build/pack/lib/scala-continuations-library_2.11-1.0.2.jar:/Users/jason/code/scala/build/pack/lib/scala-continuations-plugin_2.11.2-1.0.2.jar:/Users/jason/code/scala/build/pack/lib/scala-library.jar:/Users/jason/code/scala/build/pack/lib/scala-parser-combinators_2.11-1.0.2.jar:/Users/jason/code/scala/build/pack/lib/scala-partest-extras.jar:/Users/jason/code/scala/build/pack/lib/scala-partest-javaagent.jar:/Users/jason/code/scala/build/pack/lib/scala-reflect.jar:/Users/jason/code/scala/build/pack/lib/scala-swing_2.11-1.0.1.jar:/Users/jason/code/scala/build/pack/lib/scala-xml_2.11-1.0.2.jar:/Users/jason/code/scala/build/pack/lib/scalap.jar -classpath '""' -Dscala.home=/Users/jason/code/scala/build/pack -Dscala.usejavacp=true -Denv.emacs= scala.tools.nsc.MainGenericRunner -version

The boot classpath contains `build/pack/lib/dir` but not `foo.txt`.

I will seek a Windows test of the same during PR review.
SI-8534 Avoid crash in erroneous SelectFromTypeTree

PR #2374 changed the behaviour of `typedSingletonTypeTree` in the
presence of an error typed reference tree. It incorrectly
returns the reference tree in case on an error. However, this is
a term tree, which is an inconsistent result with the input type
tree. Consequently, a `typedSelectInternal` later fails when
using this as the qualifier of a `SelectFromTypeTree`.

Both test cases enclosed show this symptom.

This commit:

  - Returns `tree` rather than `refTyped` when `refTyped` is
    error typed or when it isn't suitable as a stable prefix.
  - Avoids issuing a cascading "not a stable prefix" error if the
    `refTyped` is error typed.
  - Adds an extra layer of defense in `typedSelectFromTypeTree`
    to bail out quickly if the qualifier is error typed.

The last measure is not necessary to fix this bug.
SI-8597 Improved pattern unchecked warnings

The spec says that `case _: List[Int]` should be always issue
an unchecked warning:

> Types which are not of one of the forms described above are
> also accepted as type patterns. However, such type patterns
> will be translated to their erasure (§3.7). The Scala compiler
> will issue an “unchecked” warning for these patterns to flag
> the possible loss of type-safety.

But the implementation goes a little further to omit warnings
based on the static type of the scrutinee. As a trivial example:

   def foo(s: Seq[Int]) = s match { case _: List[Int] => }

need not issue this warning.

These discriminating unchecked warnings are domain of
`CheckabilityChecker`.

Let's deconstruct the reported bug:

  def nowarn[T] = (null: Any) match { case _: Some[T] => }

We used to determine that if the first case matched, the scrutinee
type would be `Some[Any]` (`Some` is covariant). If this statically
matches `Some[T]` in a pattern context, we don't need to issue an
unchecked warning. But, our blanket use of `existentialAbstraction`
in `matchesPattern` loosened the pattern type to `Some[Any]`, and
the scrutinee type was deemed compatible.

I've added a new method, `scrutConformsToPatternType` which replaces
pattern type variables by wildcards, but leaves other abstract
types intact in the pattern type. We have to use this inside
`CheckabilityChecker` only. If we were to make `matchesPattern`
stricter in the same way, tests like `pos/t2486.scala` would fail.

I have introduced a new symbol test to (try to) identify pattern
type variables introduced by `typedBind`. Its not pretty, and it
might be cleaner to reserve a new flag for these.

I've also included a test variation exercising with nested matches.
The pattern type of the inner case can't, syntactically, refer to the
pattern type variable of the enclosing case. If it could, we would
have to be more selective in our wildcarding in `ptMatchesPatternType`
by restricting ourselves to type variables associated with the closest
enclosing `CaseDef`.

As some further validation of the correctness of this patch,
four stray warnings have been teased out of
neg/unchecked-abstract.scala

I also had to changes `typeArgsInTopLevelType` to extract the type
arguments of `Array[T]` if `T` is an abstract type. This avoids the
"Checkability checker says 'Uncheckable', but uncheckable type
cannot be found" warning and consequent overly lenient analysis.
Without this change, the warning was suppressed for:

    def warnArray[T] = (null: Any) match { case _: Array[T] => }
SI-8449 Fix spurious error with Java raw type over a Scala class

When the Scala typechecker typechecks signatures in Java sources,
it must enable use of a type constructor without applied type
arguments (a raw type).

However, the check in `adaptType::properTypeRequired` was too
restricive: it only allows raw types given a) we are typechecking a
Java source file and b) the type constructor itself is Java defined.

The second check does not make sense; it is perfectly legal to define
a Java raw type for over a Scala defined type constructor. This commit
removes it.

The bug was noticed in the Scaladoc context, as it explores the
signatures of all Java sources so it can document them. But the
enclosed test demonstrates the underlying bug under normal
compilation.
SI-5639 Fix spurious discarding of implicit import

In Scala fa0cdc7b (just before 2.9.0), a regression in implicit
search, SI-2866, was discovered by Lift and fixed. The nature of
the regression was that an in-scope, non-implicit symbol failed to
shadow an eponymous implicit import.

The fix for this introduced `isQualifyingImplicit` which discards
in-scope implicits when the current `Context`'s scope contains
a name-clashing entry.

Incidentally, this proved to be a shallow solution, and we later
improved shadowing detection in SI-4270 / 9129cfe9. That picked
up cases where a locally defined symbol in an intervening scope
shadowed an implicit.

This commit includes the test case from the comments of SI-2866.
Part of it is tested as a neg test (to test reporting of ambiguities),
and the rest is tested in a run test (to test which implicits are
picked.)

However, in the test case of SI-5639, we see that the scope lookup
performend by `isQualifyingImplicit` is fooled by a "ghost" module
symbol. The apparition I'm referring to is entered when
`initializeFromClassPath` / `enterClassAndModule` encounters
a class file named 'Baz.class', and speculatively enters _both_ a
class and module symbol. AFAIK, this is done to defer parsing the
class file to determine what inside. If it happens to be a Java
compiled class, the module symbol is needed to house the static
members.

This commit adds a condition that `Symbol#exists` which shines a torch
(forces the info) in the direction of the ghost module symbol. In our
test, this causes it to vanish, as we only need a class symbol for
the Scala defined `class Baz`.

The existing `pos` test for this actually did not exercise the bug,
separate compilation is required. It was originally checked in to
`pending` with this error, and then later moved to `pos` when someone
noticed it was not failing.
SI-5938 Test for a FSC bug with mixin, duplicate static forwarders

Under resident compilation, we were getting multiple copies of static
forwarders created for mixed in methods. It seems like the bug was
fixed as a by-product of #4040.

This commit adds a test to show this. I've confirmed that the test
fails appropriately with 2.11.4.

For future reference, before I figured out how to write the test
for this one (test/resident doesn't seem to let you run the code
after compiling it), I was using bash to test as follows:

    (export V=2.11.x; (scalac-hash $V sandbox/t5938_1.scala; (for i in 1 2; do echo sandbox/t5938.scala; done; printf '\n') | scalac-hash $V  -Xresident); stty echo; scala-hash $V X ; echo ':javap -public X' | scala-hash $V);
SI-8253 Fix incorrect parsing of <elem xmlns={f("a")}/>

The spliced application was placed in the `attrMap` in
`SymbolicXMLBuilder` and later incorrectly matched by a pattern
intended only to match:

    xml.Text(s)

That attribute value is generated by parsing:

   <elem xmlns='a'/>

So the net effect was that the two fragments of XML were identical!

This commit sharpens up the match to really look for a syntactic
`_root_.scala.xml.Text("...")`.

The test just prints the parse trees of a variety of cases, as we
we should not test the modularized XML library in scala/scala.
SI-6502 Refactorings suggested by review
- Moves mergeUrlsIntoClassPath from Global into ClassPath
- Revises and documents AbstractFile.getURL
SI-8932 Fix dropRight/takeRight implementations

I looked up at all the overrides of `IterableLike#takeRight` and
`IterableLike#dropRight` and replaced usages of its argument `n` with
`math.max(n, 0)` every time it was being used in an index subtraction.

Fixes [SI-8932](https://issues.scala-lang.org/browse/SI-8932).
SI-8835 Iterator tests can be junit

Without loss of generality or convenience, but helps
reduce the number of files in test/files and may reduce
compile times for test suite.

This commit includes the fix from #3963 and an extra
test of that fix that ensures the stack doesn't grow
on chained drops.