SI-8258 Revert "SI-7335 Remove special case for import of Predef._

This reverts commit 66904556ef34dc81cbb7c09257b013b3ddb76f78.

Conflicts:
    src/reflect/scala/reflect/internal/TreeInfo.scala

As evidenced by the highlights of the stack trace in Scala IDE,
my assertion in the 66904556e wasn't universally true.

The change was only motivated by removing a special case, not in
order to fix some other problem. So the revert is the most straight
forward course of action for now.

I haven't pinned this down with a test outside of Eclipse, and
given the lateness of the hour wrt 2.11.0, I'll have to submit
without one.

    2013-03-10 08:38:04,690 ERROR [main] - org.scala-ide.sdt.core - org.scala-ide.sdt.core - org.scala-ide.sdt.core - 0 - Error during askOption
    scala.reflect.internal.Symbols$CyclicReference: illegal cyclic reference involving object Predef
    ...
    at scala.reflect.internal.Symbols$Symbol.lock(Symbols.scala:482)
    at scala.reflect.internal.Symbols$Symbol.info(Symbols.scala:1216)
    at scala.reflect.internal.Symbols$Symbol.tpe(Symbols.scala:1200)
    at scala.reflect.internal.Symbols$Symbol.tpeHK(Symbols.scala:1201)
    at scala.reflect.internal.Types$Type.computeMemberType(Types.scala:784)
    at scala.reflect.internal.Types$Type.memberType(Types.scala:781)
    at scala.reflect.internal.TreeGen.mkAttributedSelect(TreeGen.scala:203)
    ...
    at scala.reflect.internal.TreeGen.mkAttributedStableRef(TreeGen.scala:162)
    at scala.tools.nsc.ast.TreeGen.mkWildcardImport(TreeGen.scala:39)
    at scala.tools.nsc.typechecker.Contexts$Context.makeNewImport(Contexts.scala:308)
    at scala.tools.nsc.typechecker.Contexts$class.rootContext(Contexts.scala:69)
    at scala.tools.nsc.Global$$anon$1.rootContext(Global.scala:492)
    at scala.tools.nsc.typechecker.Contexts$class.rootContext(Contexts.scala:64)
    at scala.tools.nsc.Global$$anon$1.rootContext(Global.scala:492)
    at scala.tools.nsc.typechecker.Analyzer$namerFactory$$anon$1.apply(Analyzer.scala:43)
    at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:463)
    ...
    at scala.tools.nsc.Global$Run.compileLate(Global.scala:1681)
    at scala.tools.nsc.Global$Run.compileLate(Global.scala:1671)
    at scala.tools.nsc.symtab.SymbolLoaders$SourcefileLoader.doComplete(SymbolLoaders.scala:284)
    at scala.tools.nsc.symtab.SymbolLoaders$SymbolLoader.complete(SymbolLoaders.scala:187)
    at scala.reflect.internal.Symbols$Symbol.info(Symbols.scala:1229)
    at scala.reflect.internal.Symbols$Symbol.tpe(Symbols.scala:1200)
    at scala.reflect.internal.Symbols$Symbol.tpeHK(Symbols.scala:1201)
    at scala.reflect.internal.Types$Type.computeMemberType(Types.scala:784)
    at scala.reflect.internal.Types$Type.memberType(Types.scala:781)
    at scala.reflect.internal.TreeGen.mkAttributedSelect(TreeGen.scala:203)
    at scala.reflect.internal.TreeGen.mkAttributedRef(TreeGen.scala:124)
    at scala.reflect.internal.TreeGen.mkAttributedRef(TreeGen.scala:130)
    at scala.reflect.internal.TreeGen.mkAttributedStableRef(TreeGen.scala:162)
    at scala.tools.nsc.ast.TreeGen.mkWildcardImport(TreeGen.scala:39)
    ...
    at scala.tools.nsc.Global$$anon$1.rootContext(Global.scala:492)
    at scala.tools.nsc.typechecker.Analyzer$namerFactory$$anon$1.apply(Analyzer.scala:43)
    at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:463)
    ...
    at scala.tools.nsc.Global$Run.compileLate(Global.scala:1671)
    at scala.tools.nsc.symtab.SymbolLoaders$SourcefileLoader.doComplete(SymbolLoaders.scala:284)
    at scala.tools.nsc.symtab.SymbolLoaders$SymbolLoader.complete(SymbolLoaders.scala:187)
    at scala.reflect.internal.Symbols$Symbol.info(Symbols.scala:1229)
    at scala.reflect.internal.Symbols$Symbol.initialize(Symbols.scala:1365)
    ...
    at scala.collection.immutable.HashSet$HashSet1.foreach(HashSet.scala:153)
    at scala.collection.immutable.HashSet$HashTrieSet.foreach(HashSet.scala:306)
    at scala.tools.eclipse.javaelements.ScalaJavaMapper$class.initializeRequiredSymbols(ScalaJavaMapper.scala:29)
    ...
    at scala.tools.nsc.util.InterruptReq.execute(InterruptReq.scala:26)
    at scala.tools.nsc.interactive.Global.pollForWork(Global.scala:340)
Start the PC thread only after initialization of required symbols.

If the class path is incomplete, the presentation compiler might crash during construction.
If the PC thread was already started, it will never get the chance to shutdown, and the
thread leaks. In the IDE, where the PC is started when needed, this can lead to a very
quick depletion of JVM threads.

See Scala IDE #1002016.
Merge pull request #3502 from retronym/ticket/8258

Fix regression for using Scala IDE on scala-library