Logging use of objects as maps for children

Let's start logging objects as maps for children. We may want to deprecate this
and replace it with ImmutableMap and Map data structures instead.

This should ideally be logged in the recursive function but since that loses the
scope of where these children are passed it's easier to start tracking them
here to get an idea of how frequently and where it's used.

src/core/ReactComponent.js

@@ -50,6 +50,7 @@ var ComponentLifeCycle = keyMirror({
 
 var ownerHasExplicitKeyWarning = {};
 var ownerHasPropertyWarning = {};
+var ownerHasMonitoredObjectMap = {};
 
 var NUMERIC_PROPERTY_REGEX = /^\d+$/;
 
@@ -152,6 +153,25 @@ function validatePropertyKey(name) {
   }
 }
 
+/**
+ * Log that we're using an object map. We're considering deprecating this
+ * feature and replace it with proper Map and ImmutableMap data structures.
+ *
+ * @internal
+ */
+function monitorUseOfObjectMap() {
+  // Name of the component whose render method tried to pass children.
+  // We only use this to avoid spewing the logs. We lose additional
+  // owner stacks but hopefully one level is enough to trace the source.
+  var currentName = (ReactCurrentOwner.current &&
+                    ReactCurrentOwner.current.constructor.displayName) || '';
+  if (ownerHasMonitoredObjectMap.hasOwnProperty(currentName)) {
+    return;
+  }
+  ownerHasMonitoredObjectMap[currentName] = true;
+  monitorCodeUse('react_object_map_children');
+}
+
 /**
  * Ensure that every component either is passed in a static location, in an
  * array with an explicit keys property defined, or in an object literal
@@ -173,6 +193,7 @@ function validateChildKeys(component) {
     // This component was passed in a valid location.
     component.__keyValidated__ = true;
   } else if (component && typeof component === 'object') {
+    monitorUseOfObjectMap();
     for (var name in component) {
       validatePropertyKey(name, component);
     }