Fix scala-js#1932: Recognize 2.12.0-M3's new shape of spurious classOf[T] trees.

The scala/scala PR scala/scala#4749 changed
the shape of the spurious `Predef.classOf[T]` that reach the
`jsinterop` phase. Previously they looked like
`scala.this.Predef.classOf[T]`, where as now they are
`scala.Predef.classOf[T]`.

This commit generalizes the shapes of trees we recognize, so that
both patterns are matched.

(cherry picked from commit eefb50a)

Author: sjrd

compiler/src/main/scala/org/scalajs/core/compiler/PrepJSInterop.scala

@@ -270,21 +270,27 @@ abstract class PrepJSInterop extends plugins.PluginComponent
           EmptyTree
         }
 
-      // Catch calls to Predef.classOf[T]. These should NEVER reach this phase
-      // but unfortunately do. In normal cases, the typer phase replaces these
-      // calls by a literal constant of the given type. However, when we compile
-      // the scala library itself and Predef.scala is in the sources, this does
-      // not happen.
-      //
-      // The trees reach this phase under the form:
-      //
-      //   scala.this.Predef.classOf[T]
-      //
-      // If we encounter such a tree, depending on the plugin options, we fail
-      // here or silently fix those calls.
-      case TypeApply(
-          classOfTree @ Select(Select(This(jstpnme.scala_), nme.Predef), nme.classOf),
-          List(tpeArg)) =>
+      /* Catch calls to Predef.classOf[T]. These should NEVER reach this phase
+       * but unfortunately do. In normal cases, the typer phase replaces these
+       * calls by a literal constant of the given type. However, when we compile
+       * the scala library itself and Predef.scala is in the sources, this does
+       * not happen.
+       *
+       * The trees reach this phase under the form:
+       *
+       *   scala.this.Predef.classOf[T]
+       *
+       * or, as of Scala 2.12.0-M3, as:
+       *
+       *   scala.Predef.classOf[T]
+       *
+       * or so it seems, at least.
+       *
+       * If we encounter such a tree, depending on the plugin options, we fail
+       * here or silently fix those calls.
+       */
+      case TypeApply(classOfTree @ Select(predef, nme.classOf), List(tpeArg))
+          if predef.symbol == PredefModule =>
         if (scalaJSOpts.fixClassOf) {
           // Replace call by literal constant containing type
           if (typer.checkClassType(tpeArg)) {