Fix scala#2636: Move last phase period assetrion

Author: nicolasstucki

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -108,9 +108,14 @@ class Compiler {
            new CollectSuperCalls,   // Find classes that are called with super
            new DropInlined,         // Drop Inlined nodes, since backend has no use for them
            new LabelDefs),          // Converts calls to labels to jumps
-      List(new GenBCode)            // Generate JVM bytecode
+      List(checkedIdOverflow(new GenBCode)) // Generate JVM bytecode
     )
 
+  private def checkedIdOverflow(phase: Phase): Phase = {
+    assert(phase.id <= Periods.MaxPossiblePhaseId)
+    phase
+  }
+
   var runId = 1
   def nextRunId = {
     runId += 1; runId

compiler/src/dotty/tools/dotc/Run.scala

@@ -24,7 +24,6 @@ import scala.util.control.NonFatal
 /** A compiler run. Exports various methods to compile source files */
 class Run(comp: Compiler)(implicit ctx: Context) {
 
-  assert(comp.phases.last.last.id <= Periods.MaxPossiblePhaseId)
   assert(ctx.runId <= Periods.MaxPossibleRunId)
 
   var units: List[CompilationUnit] = _