revert: fix($compile): throw error on invalid directive name

This reverts commit 634e467, which introduced
a breaking change between 1.3.15 and 1.3.16.

Closes #12169

Author: petebacondarwin

docs/content/error/$compile/baddir.ngdoc

@@ -757,14 +757,6 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
     return bindings;
   }
 
-  function assertValidDirectiveName(name) {
-    var letter = name.charAt(0);
-    if (!letter || letter !== lowercase(letter)) {
-      throw $compileMinErr('baddir', "Directive name '{0}' is invalid. The first character must be a lowercase letter", name);
-    }
-    return name;
-  }
-
   /**
    * @ngdoc method
    * @name $compileProvider#directive
@@ -783,7 +775,6 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
    this.directive = function registerDirective(name, directiveFactory) {
     assertNotHasOwnProperty(name, 'directive');
     if (isString(name)) {
-      assertValidDirectiveName(name);
       assertArg(directiveFactory, 'directiveFactory');
       if (!hasDirectives.hasOwnProperty(name)) {
         hasDirectives[name] = [];

src/ng/compile.js

@@ -147,7 +147,6 @@ describe('$compile', function() {
 
 
   describe('configuration', function() {
-
     it('should register a directive', function() {
       module(function() {
         directive('div', function(log) {
@@ -202,15 +201,6 @@ describe('$compile', function() {
       });
       inject(function($compile) {});
     });
-
-    it('should throw an exception if a directive name starts with a non-lowercase letter', function() {
-      module(function() {
-        expect(function() {
-          directive('BadDirectiveName', function() { });
-        }).toThrowMinErr('$compile','baddir', "Directive name 'BadDirectiveName' is invalid. The first character must be a lowercase letter");
-      });
-      inject(function($compile) {});
-    });
   });