feat(compiler): Throw a useful error message on a missing NgComponenet selector

Author: jbdeboer

lib/core_dom/selector.dart

@@ -214,11 +214,13 @@ DirectiveSelector directiveSelectorFactory(DirectiveMap directives) {
   _ElementSelector elementSelector = new _ElementSelector('');
   List<_ContainsSelector> attrSelector = [];
   List<_ContainsSelector> textSelector = [];
-
   directives.forEach((NgAnnotation annotation, Type type) {
     var match;
     var selector = annotation.selector;
     List<_SelectorPart> selectorParts;
+    if (selector == null) {
+      throw new ArgumentError('Missing selector annotation for $type');
+    }
 
     if ((match = _CONTAINS_REGEXP.firstMatch(selector)) != null) {
       textSelector.add(new _ContainsSelector(annotation, match.group(1)));

test/core_dom/compiler_spec.dart

@@ -442,6 +442,24 @@ main() => describe('dte.compiler', () {
           expect(element.textWithShadow()).toEqual('WORKED');
         })));
       });
+
+      describe('invalid components', () {
+        beforeEach(module((Module module) {
+          module
+            ..type(MissingSelector);
+          return (Injector _injector) {
+            injector = _injector;
+            $compile = injector.get(Compiler);
+            $rootScope = injector.get(Scope);
+          };
+        }));
+
+        it('should throw a useful error message', () {
+          expect(() {
+            inject((Compiler c) { });
+          }).toThrow('Missing selector annotation for MissingSelector');
+        });
+      });
     });
 
 
@@ -720,3 +738,10 @@ class MyController {
   }
 }
 
+@NgComponent(
+  template: 'boo'
+)
+class MissingSelector {
+
+}
+