fix(binding): call attach when attribute is not specified

Author: mhevery

lib/core_dom/compiler.dart

@@ -156,7 +156,7 @@ class Compiler {
           break;
         case '<=>':
           mappingFn = (NodeAttrs attrs, Scope scope, Object controller, FilterMap filters, notify()) {
-            if (attrs[attrName] == null) return;
+            if (attrs[attrName] == null) return notify();
             String expression = attrs[attrName];
             Expression expressionFn = _parser(expression);
             var blockOutbound = false;
@@ -193,7 +193,7 @@ class Compiler {
           break;
         case '=>':
           mappingFn = (NodeAttrs attrs, Scope scope, Object controller, FilterMap filters, notify()) {
-            if (attrs[attrName] == null) return;
+            if (attrs[attrName] == null) return notify();
             Expression attrExprFn = _parser(attrs[attrName]);
             var shadowValue = null;
             scope.watch(attrs[attrName],
@@ -206,7 +206,7 @@ class Compiler {
           break;
         case '=>!':
           mappingFn = (NodeAttrs attrs, Scope scope, Object controller, FilterMap filters, notify()) {
-            if (attrs[attrName] == null) return;
+            if (attrs[attrName] == null) return notify();
             Expression attrExprFn = _parser(attrs[attrName]);
             var watch;
             watch = scope.watch(

test/core_dom/compiler_spec.dart

@@ -786,7 +786,10 @@ class LogComponent {
     map: const {
         'attr-value': '@attrValue',
         'expr-value': '<=>exprValue',
-        'once-value': '=>!onceValue'
+        'once-value': '=>!onceValue',
+        'optional-one': '=>optional',
+        'optional-two': '<=>optional',
+        'optional-once': '=>!optional',
     }
 )
 class AttachDetachComponent implements NgAttachAware, NgDetachAware, NgShadowRootAware {
@@ -795,6 +798,7 @@ class AttachDetachComponent implements NgAttachAware, NgDetachAware, NgShadowRoo
   String attrValue = 'too early';
   String exprValue = 'too early';
   String onceValue = 'too early';
+  String optional;
 
   AttachDetachComponent(Logger this.logger, TemplateLoader templateLoader, Scope this.scope) {
     logger('new');