feat(directives): Do not crash on missing map parameters.

lib/block.dart

@@ -314,6 +314,7 @@ class ComponentFactory {
   createAttributeMapping(Scope parentScope, Scope shadowScope, Parser parser) {
     directive.$map.forEach((attrName, mapping) {
       var attrValue = element.attributes[attrName];
+      if (attrValue == null) attrValue = '';
       if (mapping == '@') {
         shadowScope[attrName] = attrValue;
       } else if (mapping == '=') {

test/compiler_spec.dart

@@ -392,6 +392,7 @@ main() {
       beforeEach(module((AngularModule module) {
         module.directive(SimpleComponent);
         module.directive(IoComponent);
+        module.directive(ParentExpressionComponent);
         module.directive(PublishMeComponent);
       }));
 
@@ -408,6 +409,35 @@ main() {
         }));
       }));
 
+      it('should create a component that can access parent scope', inject(() {
+        $rootScope.fromParent = "should not be used";
+        $rootScope.val = "poof";
+        var element = $('<parent-expression fromParent=val></parent-expression>');
+
+        $compile(element)(injector, element);
+
+        ParentExpressionComponent.lastTemplateLoader.template.then(expectAsync1((_) {
+          expect(renderedText(element)).toEqual('inside poof');
+        }));
+      }));
+
+      it('should behave nicely if a mapped attribute is missing', inject(() {
+        var element = $('<parent-expression></parent-expression>');
+        $compile(element)(injector, element);
+        ParentExpressionComponent.lastTemplateLoader.template.then(expectAsync1((_) {
+          expect(renderedText(element)).toEqual('inside ');
+        }));
+      }));
+
+      it('should behave nicely if a mapped attribute evals to null', inject(() {
+        $rootScope.val = null;
+        var element = $('<parent-expression fromParent=val></parent-expression>');
+        $compile(element)(injector, element);
+        ParentExpressionComponent.lastTemplateLoader.template.then(expectAsync1((_) {
+          expect(renderedText(element)).toEqual('inside ');
+        }));
+      }));
+
       it('should create a component with IO', inject(() {
         var element = $(r'<div><io attr="A" expr="name" ondone="done=true"></io></div>');
         $compile(element)(injector, element);
@@ -480,6 +510,15 @@ class IoComponent {
   }
 }
 
+class ParentExpressionComponent {
+  static String $template = '<div>inside {{fromParent()}}</div>';
+  static Map $map = {"fromParent": "&"};
+  static TemplateLoader lastTemplateLoader;
+  ParentExpressionComponent(Scope shadowScope, TemplateLoader templateLoader) {
+    lastTemplateLoader = templateLoader;
+  }
+}
+
 class PublishMeComponent {
   static String $template = r'<content>{{ctrlName.value}}</content>';
   static String $publishAs = 'ctrlName';