tools: make comma-dangle ESLint rule more stringent …

We've been having a lot of nits lately asking people to add trailing
commas, so it's probably time to incrementally make the comma-dangle
rule more stringent.

.eslintrc.js

@@ -94,7 +94,13 @@ module.exports = {
         ignorePattern: '.*',
       },
     }],
-    'comma-dangle': ['error', 'only-multiline'],
+    'comma-dangle': ['error', {
+      arrays: 'always-multiline',
+      exports: 'only-multiline',
+      functions: 'only-multiline',
+      imports: 'only-multiline',
+      objects: 'only-multiline',
+    }],
     'comma-spacing': 'error',
     'comma-style': 'error',
     'computed-property-spacing': 'error',

test/.eslintrc.yaml

@@ -5,6 +5,8 @@ env:
   es6: true
 
 rules:
+  # For now, comma-dangle is more lenient in the test directory than elsewhere.
+  comma-dangle: ["error", "only-multiline"]
   no-var: error
   prefer-const: error
   symbol-description: off