Problems reported:

parser/non-nnbd/use_required_in_non_nnbd:1:12: The modifier 'required' is only available in null safe libraries.
void foo1({required int x1}) {
           ^^^^^^^^

parser/non-nnbd/use_required_in_non_nnbd:9:12: The modifier 'required' is only available in null safe libraries.
void foo3({required required x3}) {
           ^^^^^^^^

parser/non-nnbd/use_required_in_non_nnbd:14:14: The modifier 'required' is only available in null safe libraries.
  void foo4({required covariant int x4}) {
             ^^^^^^^^

beginCompilationUnit(void)
  beginMetadataStar(void)
  endMetadataStar(0)
  beginTopLevelMember(void)
    beginTopLevelMethod(, null)
      handleVoidKeyword(void)
      handleIdentifier(foo1, topLevelFunctionDeclaration)
      handleNoTypeVariables(()
      beginFormalParameters((, MemberKind.TopLevelMethod)
        beginOptionalFormalParameters({)
          beginMetadataStar(required)
          endMetadataStar(0)
          handleRecoverableError(Message[UnexpectedModifierInNonNnbd, The modifier 'required' is only available in null safe libraries., null, {lexeme: required}], required, required)
          beginFormalParameter(int, MemberKind.TopLevelMethod, required, null, null)
            handleIdentifier(int, typeReference)
            handleNoTypeArguments(x1)
            handleType(int, null)
            handleIdentifier(x1, formalParameterDeclaration)
            handleFormalParameterWithoutValue(})
          endFormalParameter(null, null, x1, null, null, FormalParameterKind.optionalNamed, MemberKind.TopLevelMethod)
        endOptionalFormalParameters(1, {, })
      endFormalParameters(1, (, ), MemberKind.TopLevelMethod)
      handleAsyncModifier(null, null)
      beginBlockFunctionBody({)
        handleIdentifier(print, expression)
        handleNoTypeArguments(()
        beginArguments(()
          handleIdentifier(x, expression)
          handleNoTypeArguments())
          handleNoArguments())
          handleSend(x, ))
        endArguments(1, (, ))
        handleSend(print, ;)
        handleExpressionStatement(;)
      endBlockFunctionBody(1, {, })
    endTopLevelMethod(void, null, })
  endTopLevelDeclaration(void)
  beginMetadataStar(void)
  endMetadataStar(0)
  beginTopLevelMember(void)
    beginTopLevelMethod(}, null)
      handleVoidKeyword(void)
      handleIdentifier(foo2, topLevelFunctionDeclaration)
      handleNoTypeVariables(()
      beginFormalParameters((, MemberKind.TopLevelMethod)
        beginOptionalFormalParameters({)
          beginMetadataStar(required)
          endMetadataStar(0)
          beginFormalParameter(required, MemberKind.TopLevelMethod, null, null, null)
            handleIdentifier(required, typeReference)
            handleNoTypeArguments(x2)
            handleType(required, null)
            handleIdentifier(x2, formalParameterDeclaration)
            handleFormalParameterWithoutValue(})
          endFormalParameter(null, null, x2, null, null, FormalParameterKind.optionalNamed, MemberKind.TopLevelMethod)
        endOptionalFormalParameters(1, {, })
      endFormalParameters(1, (, ), MemberKind.TopLevelMethod)
      handleAsyncModifier(null, null)
      beginBlockFunctionBody({)
        handleIdentifier(print, expression)
        handleNoTypeArguments(()
        beginArguments(()
          handleIdentifier(x, expression)
          handleNoTypeArguments())
          handleNoArguments())
          handleSend(x, ))
        endArguments(1, (, ))
        handleSend(print, ;)
        handleExpressionStatement(;)
      endBlockFunctionBody(1, {, })
    endTopLevelMethod(void, null, })
  endTopLevelDeclaration(void)
  beginMetadataStar(void)
  endMetadataStar(0)
  beginTopLevelMember(void)
    beginTopLevelMethod(}, null)
      handleVoidKeyword(void)
      handleIdentifier(foo3, topLevelFunctionDeclaration)
      handleNoTypeVariables(()
      beginFormalParameters((, MemberKind.TopLevelMethod)
        beginOptionalFormalParameters({)
          beginMetadataStar(required)
          endMetadataStar(0)
          handleRecoverableError(Message[UnexpectedModifierInNonNnbd, The modifier 'required' is only available in null safe libraries., null, {lexeme: required}], required, required)
          beginFormalParameter(required, MemberKind.TopLevelMethod, required, null, null)
            handleIdentifier(required, typeReference)
            handleNoTypeArguments(x3)
            handleType(required, null)
            handleIdentifier(x3, formalParameterDeclaration)
            handleFormalParameterWithoutValue(})
          endFormalParameter(null, null, x3, null, null, FormalParameterKind.optionalNamed, MemberKind.TopLevelMethod)
        endOptionalFormalParameters(1, {, })
      endFormalParameters(1, (, ), MemberKind.TopLevelMethod)
      handleAsyncModifier(null, null)
      beginBlockFunctionBody({)
        handleIdentifier(print, expression)
        handleNoTypeArguments(()
        beginArguments(()
          handleIdentifier(x, expression)
          handleNoTypeArguments())
          handleNoArguments())
          handleSend(x, ))
        endArguments(1, (, ))
        handleSend(print, ;)
        handleExpressionStatement(;)
      endBlockFunctionBody(1, {, })
    endTopLevelMethod(void, null, })
  endTopLevelDeclaration(class)
  beginMetadataStar(class)
  endMetadataStar(0)
  beginClassOrNamedMixinApplicationPrelude(class)
    handleIdentifier(Foo, classOrMixinDeclaration)
    handleNoTypeVariables({)
    beginClassDeclaration(class, null, Foo)
      handleNoType(Foo)
      handleClassExtends(null, 1)
      handleClassNoWithClause()
      handleClassOrMixinImplements(null, 0)
      handleClassHeader(class, class, null)
      beginClassOrMixinBody(DeclarationKind.Class, {)
        beginMetadataStar(void)
        endMetadataStar(0)
        beginMember()
          beginMethod(null, null, null, null, null, foo4)
            handleVoidKeyword(void)
            handleIdentifier(foo4, methodDeclaration)
            handleNoTypeVariables(()
            beginFormalParameters((, MemberKind.NonStaticMethod)
              beginOptionalFormalParameters({)
                beginMetadataStar(required)
                endMetadataStar(0)
                handleRecoverableError(Message[UnexpectedModifierInNonNnbd, The modifier 'required' is only available in null safe libraries., null, {lexeme: required}], required, required)
                beginFormalParameter(covariant, MemberKind.NonStaticMethod, required, covariant, null)
                  handleIdentifier(int, typeReference)
                  handleNoTypeArguments(x4)
                  handleType(int, null)
                  handleIdentifier(x4, formalParameterDeclaration)
                  handleFormalParameterWithoutValue(})
                endFormalParameter(null, null, x4, null, null, FormalParameterKind.optionalNamed, MemberKind.NonStaticMethod)
              endOptionalFormalParameters(1, {, })
            endFormalParameters(1, (, ), MemberKind.NonStaticMethod)
            handleNoInitializers()
            handleAsyncModifier(null, null)
            beginBlockFunctionBody({)
              handleIdentifier(print, expression)
              handleNoTypeArguments(()
              beginArguments(()
                handleIdentifier(x, expression)
                handleNoTypeArguments())
                handleNoArguments())
                handleSend(x, ))
              endArguments(1, (, ))
              handleSend(print, ;)
              handleExpressionStatement(;)
            endBlockFunctionBody(1, {, })
          endClassMethod(null, void, (, null, })
        endMember()
      endClassOrMixinBody(DeclarationKind.Class, 1, {, })
    endClassDeclaration(class, })
  endTopLevelDeclaration()
endCompilationUnit(4, )