fix(scope): don't create own streams too eagerly

lib/core/scope.dart

@@ -161,6 +161,9 @@ class Scope {
   Scope _childHead, _childTail, _next, _prev;
   _Streams _streams;
 
+  /// Do not use. Exposes internal state for testing.
+  bool get hasOwnStreams => _streams != null  && _streams._scope == this;
+
   Scope(Object this.context, this.rootScope, this._parentScope,
         this._readWriteGroup, this._readOnlyGroup);
 
@@ -595,13 +598,17 @@ class _Streams {
   static void _forceNewScopeStream(scope, _exceptionHandler) {
     _Streams streams = scope._streams;
     Scope scopeCursor = scope;
+    bool splitMode = false;
     while(scopeCursor != null) {
       _Streams cursorStreams = scopeCursor._streams;
       var hasStream = cursorStreams != null;
       var hasOwnStream = hasStream && cursorStreams._scope == scopeCursor;
       if (hasOwnStream) return;
 
-      if (streams == null || (hasStream && !hasOwnStream)) {
+      if (!splitMode && (streams == null || (hasStream && !hasOwnStream))) {
+        if (hasStream && !hasOwnStream) {
+          splitMode = true;
+        }
         streams = new _Streams(scopeCursor, _exceptionHandler, cursorStreams);
       }
       scopeCursor._streams = streams;

test/core/scope_spec.dart

@@ -280,6 +280,46 @@ main() => describe('scope', () {
         d2.on(ScopeEvent.DESTROY).listen((_) => null);
         expect(root.apply).not.toThrow();
       }));
+
+      it('should not too eagerly create own streams', inject((RootScope root) {
+        var a = root.createChild({});
+        var a2 = root.createChild({});
+        var b = a.createChild({});
+        var c = b.createChild({});
+        var d = c.createChild({});
+        var e = d.createChild({});
+
+        getStreamState() => [root.hasOwnStreams, a.hasOwnStreams, a2.hasOwnStreams,
+                             b.hasOwnStreams, c.hasOwnStreams, d.hasOwnStreams,
+                             e.hasOwnStreams];
+
+        expect(getStreamState()).toEqual([false, false, false, false, false, false, false]);
+        expect(root.apply).not.toThrow();
+
+        e.on(ScopeEvent.DESTROY).listen((_) => null);
+        expect(getStreamState()).toEqual([false, false, false, false, false, false, true]);
+        expect(root.apply).not.toThrow();
+
+        d.on(ScopeEvent.DESTROY).listen((_) => null);
+        expect(getStreamState()).toEqual([false, false, false, false, false, true, true]);
+        expect(root.apply).not.toThrow();
+
+        b.on(ScopeEvent.DESTROY).listen((_) => null);
+        expect(getStreamState()).toEqual([false, false, false, true, false, true, true]);
+        expect(root.apply).not.toThrow();
+
+        c.on(ScopeEvent.DESTROY).listen((_) => null);
+        expect(getStreamState()).toEqual([false, false, false, true, true, true, true]);
+        expect(root.apply).not.toThrow();
+
+        a.on(ScopeEvent.DESTROY).listen((_) => null);
+        expect(getStreamState()).toEqual([false, true, false, true, true, true, true]);
+        expect(root.apply).not.toThrow();
+
+        a2.on(ScopeEvent.DESTROY).listen((_) => null);
+        expect(getStreamState()).toEqual([true, true, true, true, true, true, true]);
+        expect(root.apply).not.toThrow();
+      }));
     });