feat(Scope): Use VmTurnZone.onScheduleMicrotask in Scope

BREAKING CHANGE:

Previously a micro task registered in flush phase would cause a new
digest cycle after the current digest cycle. The new behavior
will cause an error.

Closes #984

Author: mvuksano

lib/core/scope.dart

@@ -644,6 +644,7 @@ class RootScope extends Scope {
   {
     _zone.onTurnDone = apply;
     _zone.onError = (e, s, ls) => _exceptionHandler(e, s);
+    _zone.onScheduleMicrotask = runAsync;
   }
 
   RootScope get rootScope => this;
@@ -787,6 +788,9 @@ class RootScope extends Scope {
 
   // QUEUES
   void runAsync(fn()) {
+    if (_state == STATE_FLUSH || _state == STATE_FLUSH_ASSERT) {
+      throw "Scheduling microtasks not allowed in $state state.";
+    }
     var chain = new _FunctionChain(fn);
     if (_runAsyncHead == null) {
       _runAsyncHead = _runAsyncTail = chain;

lib/core/zone.dart

@@ -60,6 +60,7 @@ class VmTurnZone {
   /// an "inner" [Zone], which is a child of the outer [Zone].
   async.Zone _innerZone;
 
+
   /**
    * Associates with this
    *

test/core/scope_spec.dart

@@ -1440,6 +1440,98 @@ void main() {
       });
     });
 
+    describe('microtask processing', () {
+      beforeEach((VmTurnZone zone, RootScope scope, Logger log) {
+        var onTurnDone = zone.onTurnDone;
+        zone.onTurnDone = () {
+          log('[');
+          onTurnDone();
+          log(']');
+        };
+        var onScheduleMicrotask = zone.onScheduleMicrotask;
+        zone.onScheduleMicrotask = (fn) {
+          log('(');
+          try {
+            onScheduleMicrotask(fn);
+          } catch (e) {
+            log('CATCH: $e');
+          }
+          log(')');
+        };
+      });
+
+      it('should schedule apply after future resolution',
+        async((Logger log, VmTurnZone zone, RootScope scope) {
+          Completer completer;
+          zone.run(() {
+            completer = new Completer();
+            completer.future.then((value) {
+              log('then($value)');
+            });
+          });
+
+          scope.runAsync(() => log('before'));
+          log.clear();
+          completer.complete('OK'); // this one causes APPLY which processe 'before'
+          // This one schedules work but apply already run so it does not execute.
+          scope.runAsync(() => log('NOT_EXECUTED'));
+
+          expect(log).toEqual(['(', ')', '[', 'before', 'then(OK)', ']']);
+        })
+      );
+
+      it('should schedule microtask to runAsync queue during digest',
+        async((Logger log, VmTurnZone zone, RootScope scope) {
+          Completer completer;
+          zone.run(() {
+            completer = new Completer();
+            completer.future.
+              then((value) {
+                scope.runAsync(() => log('in(${scope.state})'));
+                return new Future.value(value);
+              }).
+              then((value) {
+                log('then($value)');
+              });
+          });
+          log.clear();
+          completer.complete('OK');
+          expect(log).toEqual(['(', ')', '[', '(', ')', 'in(digest)', 'then(OK)', ']']);
+        })
+      );
+
+      it('should not allow microtasks in flush phase',
+        async((Logger log, VmTurnZone zone, RootScope scope) {
+          zone.run(() {
+            scope.domWrite(() {
+              return new Future.value('FAIL');
+            });
+          });
+          expect(log).toEqual(
+              ['[', '(', 'CATCH: Scheduling microtasks not allowed in flush state.', ')', ']']);
+        })
+      );
+
+      it('should allow creation of Completers in flush phase',
+        async((Logger log, VmTurnZone zone, RootScope scope) {
+          Completer completer;
+          zone.run(() {
+            scope.domWrite(() {
+              log('new Completer');
+              completer = new Completer();
+              completer.future.then((value) {
+                log('then($value)');
+              });
+            });
+          });
+          log('=');
+          completer.complete('OK');
+          log(';');
+          expect(log).toEqual(
+              ['[', 'new Completer', ']', '=', '(', ')', '[', 'then(OK)', ']', ';']);
+        })
+      );
+    });
 
     describe('domRead/domWrite', () {
       beforeEachModule((Module module) {