fix(scope): allow watching an empty string

lib/core/scope.dart

@@ -205,17 +205,21 @@ class Scope {
     assert(expression is String);
     Watch watch;
     ReactionFn fn = reactionFn;
-    if (expression.startsWith('::')) {
-      expression = expression.substring(2);
-      fn = (value, last) {
-        if (value != null) {
-          watch.remove();
-          return reactionFn(value, last);
-        }
-      };
-    } else if (expression.startsWith(':')) {
-      expression = expression.substring(1);
-      fn = (value, last) => value == null ? null : reactionFn(value, last);
+    if (expression.isEmpty) {
+      expression = '""';
+    } else {
+      if (expression.startsWith('::')) {
+        expression = expression.substring(2);
+        fn = (value, last) {
+          if (value != null) {
+            watch.remove();
+            return reactionFn(value, last);
+          }
+        };
+      } else if (expression.startsWith(':')) {
+        expression = expression.substring(1);
+        fn = (value, last) => value == null ? null : reactionFn(value, last);
+      }
     }
 
     AST ast = rootScope._astParser(expression, context: context,

test/core/scope_spec.dart

@@ -66,12 +66,14 @@ void main() {
       it('should watch literals', (Logger logger, Map context, RootScope rootScope) {
         context['a'] = 1;
         rootScope
+            ..watch('', (value, previous) => logger(value))
+            ..watch('""', (value, previous) => logger(value))
             ..watch('1', (value, previous) => logger(value))
             ..watch('"str"', (value, previous) => logger(value))
             ..watch('[a, 2, 3]', (value, previous) => logger(value))
             ..watch('{a:a, b:2}', (value, previous) => logger(value))
             ..digest();
-        expect(logger).toEqual([1, 'str', [1, 2, 3], {'a': 1, 'b': 2}]);
+        expect(logger).toEqual(['', '', 1, 'str', [1, 2, 3], {'a': 1, 'b': 2}]);
         logger.clear();
         context['a'] = 3;
         rootScope.digest();