feat($compile): Throw error on incorrect casing

- Adds error message for when the first letter of the directive name is
  not capitalized in the definition

Author: wesleycho

src/ng/compile.js

@@ -779,6 +779,14 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
     return bindings;
   }
 
+  function assertFirstLetterIsLowerCased(name) {
+    var letter = name.charAt(0);
+    if (letter !== letter.toLowerCase()) {
+      throw ngMinErr('badname', "Directive '{0}' is not a valid name. Make sure the first letter is lowercase");
+    }
+    return name;
+  }
+
   /**
    * @ngdoc method
    * @name $compileProvider#directive
@@ -797,6 +805,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
    this.directive = function registerDirective(name, directiveFactory) {
     assertNotHasOwnProperty(name, 'directive');
     if (isString(name)) {
+      assertFirstLetterIsLowerCased(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) {
@@ -7430,3 +7431,19 @@ describe('$compile', function() {
       }));
   });
 });
+
+describe('$compile', function() {
+  describe('directive', function() {
+    it('should fail to register a directive', function() {
+      try {
+        module(provideLog, function($provide, $compileProvider) {
+          $compileProvider.directive('BadDirectiveName', function(log) {
+            return {};
+          });
+        });
+      } catch (e) {
+        expect(e).toBe("Directive 'BadDirectiveName' is not a valid name. Make sure the first letter is lowercase")
+      }
+    });
+  });
+});