#5520, throw error when no dialect supplied

lib/sequelize.js

@@ -149,7 +149,7 @@ var Sequelize = function(database, username, password, options) {
   Sequelize.runHooks('beforeInit', config, options);
 
   this.options = Utils._.extend({
-    dialect: 'mysql',
+    dialect: null,
     dialectModulePath: null,
     host: 'localhost',
     protocol: 'tcp',
@@ -173,6 +173,10 @@ var Sequelize = function(database, username, password, options) {
     benchmark: false
   }, options || {});
 
+  if (!this.options.dialect) {
+    throw new Error('Dialect needs to be explicitly supplied as of v4.0.0');
+  }
+
   if (this.options.dialect === 'postgresql') {
     this.options.dialect = 'postgres';
   }