Problems reported:

parser/nnbd/issue_40267_case_04:4:23: Expected ':' before this.
    case x?[4]:{{{2}}}; // as case (x?[4]:{{{2}}}) [missing :]
                      ^

beginCompilationUnit(main)
  beginMetadataStar(main)
  endMetadataStar(0)
  beginTopLevelMember(main)
    beginTopLevelMethod(, null)
      handleNoType()
      handleIdentifier(main, topLevelFunctionDeclaration)
      handleNoTypeVariables(()
      beginFormalParameters((, MemberKind.TopLevelMethod)
      endFormalParameters(0, (, ), MemberKind.TopLevelMethod)
      handleAsyncModifier(null, null)
      beginBlockFunctionBody({)
        beginSwitchStatement(switch)
          handleIdentifier(a, expression)
          handleNoTypeArguments())
          handleNoArguments())
          handleSend(a, ))
          handleParenthesizedCondition(()
          beginSwitchBlock({)
            beginCaseExpression(case)
              handleIdentifier(x, expression)
              handleNoTypeArguments(?)
              handleNoArguments(?)
              handleSend(x, ?)
              beginConditionalExpression(?)
                handleNoTypeArguments([)
                handleLiteralInt(4)
                handleLiteralList(1, [, null, ])
                handleConditionalExpressionColon()
                handleNoTypeArguments({)
                handleNoTypeArguments({)
                handleNoTypeArguments({)
                handleLiteralInt(2)
                handleLiteralSetOrMap(1, {, null, }, true)
                handleLiteralSetOrMap(1, {, null, }, true)
                handleLiteralSetOrMap(1, {, null, }, true)
              endConditionalExpression(?, :)
              handleRecoverableError(Message[ExpectedButGot, Expected ':' before this., null, {string: :}], ;, ;)
            endCaseExpression(:)
            handleCaseMatch(case, :)
            beginSwitchCase(0, 1, case)
              handleEmptyStatement(;)
            endSwitchCase(0, 1, null, null, 1, case, })
          endSwitchBlock(1, {, })
        endSwitchStatement(switch, })
      endBlockFunctionBody(1, {, })
    endTopLevelMethod(main, null, })
  endTopLevelDeclaration()
endCompilationUnit(1, )