Add an assert that we don't schedule a render during render

In production I added an early return.  I don't bother to log
in production since no one is looking at the log anyway.

Unfortunately this currently only fires when using a debug build
we should fix our Release build to have a checked-mode option
or consider hacks like having all localhost urls enable checked
mode, etc.

R=ianh@google.com, rafaelw@chromium.org
BUG=

Review URL: https://codereview.chromium.org/983973005

framework/fn.dart

@@ -503,24 +503,34 @@ class Anchor extends Element {
 
 List<Component> _dirtyComponents = new List<Component>();
 bool _renderScheduled = false;
+bool _inRenderDirtyComponents = false;
 
 void _renderDirtyComponents() {
-  Stopwatch sw = new Stopwatch()..start();
+  try {
+    _inRenderDirtyComponents = true;
+    Stopwatch sw = new Stopwatch()..start();
 
-  _dirtyComponents.sort((a, b) => a._order - b._order);
-  for (var comp in _dirtyComponents) {
-    comp._renderIfDirty();
-  }
+    _dirtyComponents.sort((a, b) => a._order - b._order);
+    for (var comp in _dirtyComponents) {
+      comp._renderIfDirty();
+    }
 
-  _dirtyComponents.clear();
-  _renderScheduled = false;
+    _dirtyComponents.clear();
+    _renderScheduled = false;
 
-  sw.stop();
-  if (_shouldLogRenderDuration)
-    print("Render took ${sw.elapsedMicroseconds} microseconds");
+    sw.stop();
+    if (_shouldLogRenderDuration)
+      print("Render took ${sw.elapsedMicroseconds} microseconds");
+  } finally {
+    _inRenderDirtyComponents = false;
+  }
 }
 
 void _scheduleComponentForRender(Component c) {
+  assert(!_inRenderDirtyComponents);
+  if (_inRenderDirtyComponents)
+    return;
+
   _dirtyComponents.add(c);
 
   if (!_renderScheduled) {