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

Directive names must start with a lower case letter.
Previously the compiler would quietly fail.
This change adds an assertion that fails if this is not the case.

Closes #11281
Closes #11109

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

@@ -0,0 +1,8 @@
+@ngdoc error
+@name $compile:baddir
+@fullName Invalid Directive Name
+@description
+
+This error occurs when the name of a directive is not valid.
+
+Directives must start with a lowercase character.

src/ng/compile.js

@@ -790,6 +790,14 @@ 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
@@ -808,6 +816,7 @@ 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] = [];

test/ng/compileSpec.js

@@ -147,6 +147,7 @@ describe('$compile', function() {
 
 
   describe('configuration', function() {
+
     it('should register a directive', function() {
       module(function() {
         directive('div', function(log) {
@@ -201,6 +202,15 @@ 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) {});
+    });
   });