fix(NgStyle): make NgStyle export expressions

Closes #993

Author: vicb

Diff for: lib/directive/ng_style.dart

@@ -1,15 +1,16 @@
 part of angular.directive;
 
 /**
-  * The `ngStyle` directive allows you to set CSS style on an HTML element
-  * conditionally.
-  *
-  * @example
-  *     <span ng-style="{color:'red'}">Sample Text</span>
-  */
+ * The `NgStyle` directive allows you to set CSS style on an HTML element conditionally.
+ *
+ * # example
+ *
+ *     <span ng-style="{color:'red'}">Sample Text</span>
+ */
 @Decorator(
     selector: '[ng-style]',
-    map: const {'ng-style': '@styleExpression'})
+    map: const {'ng-style': '@styleExpression'},
+    exportExpressionAttrs: const ['ng-style'])
 class NgStyle {
   final dom.Element _element;
   final Scope _scope;
@@ -19,7 +20,7 @@ class NgStyle {
 
   NgStyle(this._element, this._scope);
 
-/**
+ /**
   * ng-style attribute takes an expression which evaluates to an
   * object whose keys are CSS style names and values are corresponding values
   * for those CSS keys.