eslint rule to make sure imports are on a single line, see phetsims/p…

…het-info#139

eslint/.eslintrc.js

@@ -76,6 +76,9 @@ module.exports = {
     // Custom rule for avoiding instanceof Array.
     'no-instanceof-array': 2,
 
+    // Custom rule for keeping import statements on a single line.
+    'single-line-import': 2,
+
     // disallow declaration of variables that are not used in the code (recommended)
     // Overriden to allow unused args
     'no-unused-vars': [

eslint/rules/single-line-import.js

@@ -0,0 +1,51 @@
+// Copyright 2016-2020, University of Colorado Boulder
+/**
+ * @fileoverview Rule to check that import statements are on single lines. Automated tools
+ * and processes at PhET assume that imports are on a single line so this is important to enforce.
+ *
+ * @author Jesse Greenberg (PhET Interactive Simulations)
+ * @copyright 2020 University of Colorado Boulder
+ */
+
+/* eslint-env node */
+'use strict';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = context => {
+
+  return {
+
+    ImportDeclaration: node => {
+      if ( node.loc.start.line !== node.loc.end.line ) {
+
+        // AST JSON might look something like:
+        // {
+        //   "type": "ImportDeclaration",
+        //   "specifiers": [
+        //     {
+        //       "type": "ImportDefaultSpecifier",
+        //       "local": {
+        //         "type": "Identifier",
+        //         "name": "EnergySkateParkColorScheme",
+        //       }
+        //     }
+        //   ]
+        // }
+        node.specifiers.forEach( specifier => {
+          context.report( {
+            node: node,
+            loc: node.loc,
+            message: specifier.local.name + ': import statement should be on a single line.'
+          } );
+        } );
+      }
+    }
+  };
+};
+
+module.exports.schema = [
+  // JSON Schema for rule options goes here
+];