Problems reported:

parser/error_recovery/keyword_named_formal_parameter_start_of_next_top_level:2:1: Expected a function body, but got 'class'.
class C {}
^^^^^

parser/error_recovery/keyword_named_formal_parameter_start_of_next_top_level:1:2: Can't find ')' to match '('.
f(x /* guess the user was typing here... */
 ^

beginCompilationUnit(f)
  beginMetadataStar(f)
  endMetadataStar(0)
  beginTopLevelMember(f)
    beginTopLevelMethod(UnmatchedToken((), null)
      handleNoType(UnmatchedToken(())
      handleIdentifier(f, topLevelFunctionDeclaration)
      handleNoTypeVariables(()
      beginFormalParameters((, MemberKind.TopLevelMethod)
        beginMetadataStar(x)
        endMetadataStar(0)
        beginFormalParameter(x, MemberKind.TopLevelMethod, null, null, null)
          handleNoType(()
          handleIdentifier(x, formalParameterDeclaration)
          handleFormalParameterWithoutValue(class)
        endFormalParameter(null, null, null, x, null, null, FormalParameterKind.mandatory, MemberKind.TopLevelMethod)
      endFormalParameters(1, (, ), MemberKind.TopLevelMethod)
      handleAsyncModifier(null, null)
      handleRecoverableError(Message[ExpectedFunctionBody, Expected a function body, but got 'class'., null, {lexeme: class}], class, class)
      handleInvalidFunctionBody({)
    endTopLevelMethod(f, null, })
  endTopLevelDeclaration(class)
  beginMetadataStar(class)
  endMetadataStar(0)
  beginClassOrMixinOrNamedMixinApplicationPrelude(class)
    handleIdentifier(C, classOrMixinDeclaration)
    handleNoTypeVariables({)
    beginClassDeclaration(class, null, C)
      handleNoType(C)
      handleClassExtends(null, 1)
      handleClassNoWithClause()
      handleClassOrMixinImplements(null, 0)
      handleClassHeader(class, class, null)
      beginClassOrMixinOrExtensionBody(DeclarationKind.Class, {)
      endClassOrMixinOrExtensionBody(DeclarationKind.Class, 0, {, })
    endClassDeclaration(class, })
  endTopLevelDeclaration()
  handleErrorToken(UnmatchedToken(())
  handleRecoverableError(Message[UnmatchedToken, Can't find ')' to match '('., null, {string: ), lexeme: (}], UnmatchedToken((), UnmatchedToken(())
endCompilationUnit(2, )