fix(watch_group): fix for NaN !== NaN

lib/change_detection/ast.dart

@@ -91,8 +91,7 @@ class PureFunctionAST extends AST {
 /**
  * SYNTAX: fn(arg0, arg1, ...)
  *
- * Invoke a pure function. Pure means that the function has no state, and
- * therefore it needs to be re-computed only if its args change.
+ * Invoke a (non-pure) function.
  */
 class ClosureAST extends AST {
   final String name;

lib/change_detection/watch_group.dart

@@ -849,6 +849,8 @@ class _EvalWatchRecord implements WatchRecord<_Handler> {
       if (value is String && current is String && value == current) {
         // it is really the same, recover and save so next time identity is same
         current = value;
+      } else if (value is num && value.isNaN && current is num && current.isNaN) {
+        // we need this for the compiled JavaScript since in JS NaN !== NaN.
       } else {
         previousValue = current;
         currentValue = value;

test/change_detection/watch_group_spec.dart

@@ -528,6 +528,27 @@ void main() {
         expect(logger).toEqual([]);
       });
 
+      it('should ignore NaN != NaN', () {
+        watchGrp.watch(new ClosureAST('NaN', () => double.NAN, []), (_, __) => logger('NaN'));
+
+        watchGrp.detectChanges();
+        expect(logger).toEqual(['NaN']);
+
+        logger.clear();
+        watchGrp.detectChanges();
+        expect(logger).toEqual([]);
+      }) ;
+
+      it('should test string by value', () {
+        watchGrp.watch(new ClosureAST('String', () => 'value', []), (v, _) => logger(v));
+
+        watchGrp.detectChanges();
+        expect(logger).toEqual(['value']);
+
+        logger.clear();
+        watchGrp.detectChanges();
+        expect(logger).toEqual([]);
+      });
 
       it('should eval method', () {
         var obj = new MyClass(logger);