Fix Mixin: remove Module flag in mixin synthesis

Module flag will cause the companion class defined in a trait
to be incorrectly entered as a member of the class that
implement the trait.

The method `Symbol.entered` has the logic to enter the companion class:

    if (this.is(Module)) this.owner.asClass.enter(this.moduleClass)

The following is a simple test case:

    trait A{
      object O
    }

    class B extends A {
      def foo = O
    }

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/MixinOps.scala

@@ -22,7 +22,7 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(implicit ctx: Cont
     val res = member.copy(
       owner = cls,
       name = member.name.stripScala2LocalSuffix,
-      flags = member.flags &~ Deferred | Synthetic | extraFlags,
+      flags = member.flags &~ Deferred &~ Module | Synthetic | extraFlags,
       info = cls.thisType.memberInfo(member)).enteredAfter(thisPhase).asTerm
     res.addAnnotations(member.annotations.filter(_.symbol != defn.TailrecAnnot))
     res

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -427,18 +427,18 @@ class TreeChecker extends Phase with SymTransformer {
       checkOwner(impl.constr)
 
       def isNonMagicalMember(x: Symbol) =
-          !x.isValueClassConvertMethod &&
-          !x.name.is(DocArtifactName)
+        !x.isValueClassConvertMethod &&
+        !x.name.is(DocArtifactName)
 
       val decls   = cls.classInfo.decls.toList.toSet.filter(isNonMagicalMember)
       val defined = impl.body.map(_.symbol)
 
       val symbolsNotDefined = decls -- defined - constr.symbol
 
       assert(symbolsNotDefined.isEmpty,
-          i" $cls tree does not define methods: ${symbolsNotDefined.toList}%, %\n" +
-          i"expected: $decls%, %\n" +
-          i"defined: $defined%, %")
+          i" $cls tree does not define members: ${symbolsNotDefined.toList}%, %\n" +
+          i"expected: ${decls.toList}%, %\n" +
+          i"defined: ${defined}%, %")
 
       super.typedClassDef(cdef, cls)
     }