Fix: add comma-style handling of lone commas when destructuring arrays

This handles ignored elements as per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#Ignoring_some_returned_values

Closes eslint#12756

lib/rules/comma-style.js

@@ -124,10 +124,11 @@ module.exports = {
          * @param {Token} commaToken The token representing the comma.
          * @param {Token} currentItemToken The first token of the current item.
          * @param {Token} reportItem The item to use when reporting an error.
+         * @param {boolean} arrayPattern whether the comma is in a destructuring assignment.
          * @returns {void}
          * @private
          */
-        function validateCommaItemSpacing(previousItemToken, commaToken, currentItemToken, reportItem) {
+        function validateCommaItemSpacing(previousItemToken, commaToken, currentItemToken, reportItem, arrayPattern) {
 
             // if single line
             if (astUtils.isTokenOnSameLine(commaToken, currentItemToken) &&
@@ -144,15 +145,19 @@ module.exports = {
                     : "between";
 
                 // lone comma
-                context.report({
-                    node: reportItem,
-                    loc: {
-                        line: commaToken.loc.end.line,
-                        column: commaToken.loc.start.column
-                    },
-                    messageId: "unexpectedLineBeforeAndAfterComma",
-                    fix: getFixerFunction(styleType, previousItemToken, commaToken, currentItemToken)
-                });
+                if (arrayPattern) {
+                    // ignored element, move on.
+                } else {
+                    context.report({
+                        node: reportItem,
+                        loc: {
+                            line: commaToken.loc.end.line,
+                            column: commaToken.loc.start.column
+                        },
+                        messageId: "unexpectedLineBeforeAndAfterComma",
+                        fix: getFixerFunction(styleType, previousItemToken, commaToken, currentItemToken)
+                    });
+                }
 
             } else if (style === "first" && !astUtils.isTokenOnSameLine(commaToken, currentItemToken)) {
 
@@ -185,9 +190,10 @@ module.exports = {
          */
         function validateComma(node, property) {
             const items = node[property],
-                arrayLiteral = (node.type === "ArrayExpression" || node.type === "ArrayPattern");
+                arrayLiteral = node.type === "ArrayExpression",
+                arrayPattern = node.type === "ArrayPattern";
 
-            if (items.length > 1 || arrayLiteral) {
+            if (items.length > 1 || arrayLiteral || arrayPattern) {
 
                 // seed as opening [
                 let previousItemToken = sourceCode.getFirstToken(node);
@@ -213,7 +219,7 @@ module.exports = {
                      */
                     if (astUtils.isCommaToken(commaToken)) {
                         validateCommaItemSpacing(previousItemToken, commaToken,
-                            currentItemToken, reportItem);
+                            currentItemToken, reportItem, arrayPattern);
                     }
 
                     if (item) {
@@ -226,13 +232,12 @@ module.exports = {
                 });
 
                 /*
-                 * Special case for array literals that have empty last items, such
+                 * Special case for arrays that have empty last items, such
                  * as [ 1, 2, ]. These arrays only have two items show up in the
                  * AST, so we need to look at the token to verify that there's no
                  * dangling comma.
                  */
-                if (arrayLiteral) {
-
+                if (arrayLiteral || arrayPattern) {
                     const lastToken = sourceCode.getLastToken(node),
                         nextToLastToken = sourceCode.getTokenBefore(lastToken);