fix(model): direct access array with multiple checkboxes

Closes vuejs#7670

Author: posva

src/platforms/web/compiler/directives/model.js

@@ -86,8 +86,8 @@ function genCheckboxModel (
     'if(Array.isArray($$a)){' +
       `var $$v=${number ? '_n(' + valueBinding + ')' : valueBinding},` +
           '$$i=_i($$a,$$v);' +
-      `if($$el.checked){$$i<0&&(${value}=$$a.concat([$$v]))}` +
-      `else{$$i>-1&&(${value}=$$a.slice(0,$$i).concat($$a.slice($$i+1)))}` +
+      `if($$el.checked){$$i<0&&(${genAssignmentCode(value, '$$a.concat([$$v])')})}` +
+      `else{$$i>-1&&(${genAssignmentCode(value, '$$a.slice(0,$$i).concat($$a.slice($$i+1))')})}` +
     `}else{${genAssignmentCode(value, '$$c')}}`,
     null, true
   )