Add rule to enforce one provide per file

package.json

@@ -100,6 +100,7 @@
       "openlayers-internal/no-duplicate-requires": 2,
       "openlayers-internal/no-missing-requires": 1,
       "openlayers-internal/no-unused-requires": 2,
+      "openlayers-internal/one-provide": 1,
       "openlayers-internal/requires-first": 2,
       "openlayers-internal/valid-requires": 2
     }

rules/index.js

@@ -5,6 +5,7 @@ module.exports = {
     'no-duplicate-requires': require('./no-duplicate-requires').rule,
     'no-missing-requires': require('./no-missing-requires').rule,
     'no-unused-requires': require('./no-unused-requires').rule,
+    'one-provide': require('./one-provide').rule,
     'requires-first': require('./requires-first').rule,
     'valid-requires': require('./valid-requires').rule
   }

rules/one-provide.js

@@ -0,0 +1,28 @@
+'use strict';
+
+const util = require('./util');
+
+exports.rule = {
+  meta: {
+    docs: {
+      description: 'disallow multiple goog.provide() calls'
+    }
+  },
+
+  create: function(context) {
+    let hasProvide = false;
+
+    return {
+      ExpressionStatement: function(statement) {
+        if (util.isProvideStatement(statement)) {
+          if (hasProvide) {
+            const name = statement.expression.arguments[0].value;
+            context.report(statement, `Extra goog.provide('${name}')`);
+          } else {
+            hasProvide = true;
+          }
+        }
+      }
+    };
+  }
+};