feat(change_detection): treat List properties as pure

lib/change_detection/change_detection.dart

@@ -55,7 +55,8 @@ abstract class ChangeDetector<H> extends ChangeDetectorGroup<H> {
    * same order as they were registered.
    */
   Iterator<Record<H>> collectChanges({ EvalExceptionHandler exceptionHandler,
-                                   AvgStopwatch stopwatch });
+                                       AvgStopwatch stopwatch,
+                                       bool reMemoizeForPureFunc });
 }
 
 abstract class Record<H> {

lib/change_detection/dirty_checking_change_detector.dart

@@ -282,13 +282,14 @@ class DirtyCheckingChangeDetector<H> extends DirtyCheckingChangeDetectorGroup<H>
   }
 
   Iterator<Record<H>> collectChanges({EvalExceptionHandler exceptionHandler,
-                                      AvgStopwatch stopwatch}) {
+                                      AvgStopwatch stopwatch, bool reMemoizeForPureFunc}) {
     if (stopwatch != null) stopwatch.start();
     DirtyCheckingRecord changeTail = _fakeHead;
     DirtyCheckingRecord current = _recordHead; // current index
 
     int count = 0;
     while (current != null) {
+      if (reMemoizeForPureFunc == null || reMemoizeForPureFunc) current.dirtyArgs = true;
       try {
         if (current.check()) changeTail = changeTail._nextChange = current;
         count++;
@@ -373,6 +374,7 @@ class DirtyCheckingRecord<H> implements Record<H>, WatchRecord<H> {
   Record<H> _nextChange;
   var _object;
   InstanceMirror _instanceMirror;
+  bool dirtyArgs = true;
 
   DirtyCheckingRecord(this._group, object, fieldName, this._getter, this.handler)
       : field = fieldName,
@@ -443,7 +445,9 @@ class DirtyCheckingRecord<H> implements Record<H>, WatchRecord<H> {
       case _MODE_MARKER_:
         return false;
       case _MODE_REFLECT_:
+        if (!dirtyArgs) return false;
         current = _instanceMirror.getField(_symbol).reflectee;
+        if (object is List) dirtyArgs = false;
         break;
       case _MODE_GETTER_:
         current = _getter(object);

lib/change_detection/watch_group.dart

@@ -370,12 +370,13 @@ class RootWatchGroup extends WatchGroup {
                       ChangeLog changeLog,
                       AvgStopwatch fieldStopwatch,
                       AvgStopwatch evalStopwatch,
-                      AvgStopwatch processStopwatch}) {
+                      AvgStopwatch processStopwatch, bool reMemoizeForPureFunc : true}) {
     // Process the Records from the change detector
     Iterator<Record<_Handler>> changedRecordIterator =
         (_changeDetector as ChangeDetector<_Handler>).collectChanges(
             exceptionHandler:exceptionHandler,
-            stopwatch: fieldStopwatch);
+            stopwatch: fieldStopwatch,
+            reMemoizeForPureFunc: reMemoizeForPureFunc);
     if (processStopwatch != null) processStopwatch.start();
     while (changedRecordIterator.moveNext()) {
       var record = changedRecordIterator.current;

lib/core/scope.dart

@@ -467,7 +467,8 @@ class RootScope extends Scope {
             changeLog: changeLog,
             fieldStopwatch: _scopeStats.digestFieldStopwatch,
             evalStopwatch: _scopeStats.digestEvalStopwatch,
-            processStopwatch: _scopeStats.digestProcessStopwatch);
+            processStopwatch: _scopeStats.digestProcessStopwatch,
+            reMemoizeForPureFunc: count == null);
 
         if (digestTTL <= LOG_COUNT) {
           if (changeLog == null) {

test/core/scope_spec.dart

@@ -161,6 +161,19 @@ void main() {
         rootScope.digest();
         expect(logger).toEqual([true]);
       });
+
+      it('should watch list property as pure', (Logger logger, Map context, RootScope rootScope) {
+        var list = [true, 2, 'abc'];
+        final logVal = (value, _) => logger(value);
+        context['list'] = list;
+        rootScope.watch('list.reversed', logVal);
+        rootScope.digest();
+        expect(logger).toEqual([['abc', 2, true]]);
+        logger.clear();
+        context['list'][2] = 'def';
+        rootScope.digest();
+        expect(logger).toEqual([['def', 2, true]]);
+      });
 
       it('should support filters', (Logger logger, Map context,
                                            RootScope rootScope, AstParser parser,