Start warning for JSX on plain functions

Let's also bump the version number for the log so that we can start filtering
out the other logs.

src/core/ReactLegacyDescriptor.js

@@ -65,15 +65,16 @@ function warnForPlainFunctionType(type) {
       } catch (x) {
         // just incase this is a frozen object or some special object
       }
-      monitorCodeUse('react_non_component_in_jsx', { name: type.name });
+      monitorCodeUse(
+        'react_non_component_in_jsx',
+        { version: 2, name: type.name }
+      );
     }
-    // TODO: This pattern is heavily used by ReactMenu and therefore we
-    // cannot yet warn without spamming users too much.
-    // warning(
-    //   false,
-    //   'This JSX uses a plain function. Only React components are ' +
-    //   'valid in React\'s JSX transform.'
-    // );
+    warning(
+      false,
+      'This JSX uses a plain function. Only React components are ' +
+      'valid in React\'s JSX transform.'
+    );
   }
 }
 

src/core/__tests__/ReactDescriptor-test.js

@@ -248,11 +248,10 @@ describe('ReactDescriptor', function() {
       return 21 + x;
     });
     expect(factory(21)).toBe(42);
-    // TODO: This warning is temporarily disabled
-    expect(console.warn.argsForCall.length).toBe(0);
-    // expect(console.warn.argsForCall[0][0]).toContain(
-    //   'This JSX uses a plain function.'
-    // );
+    expect(console.warn.argsForCall.length).toBe(1);
+    expect(console.warn.argsForCall[0][0]).toContain(
+      'This JSX uses a plain function.'
+    );
   });
 
   it('warns but allow a plain function to be immediately invoked', function() {
@@ -261,11 +260,10 @@ describe('ReactDescriptor', function() {
       return 21 + x + y;
     }, 11, 10);
     expect(result).toBe(42);
-    // TODO: This warning is temporarily disabled
-    expect(console.warn.argsForCall.length).toBe(0);
-    // expect(console.warn.argsForCall[0][0]).toContain(
-    //   'This JSX uses a plain function.'
-    // );
+    expect(console.warn.argsForCall.length).toBe(1);
+    expect(console.warn.argsForCall[0][0]).toContain(
+      'This JSX uses a plain function.'
+    );
   });
 
   it('warns but does not fail on undefined results', function() {
@@ -273,11 +271,10 @@ describe('ReactDescriptor', function() {
     var fn = function () { };
     var result = React.createDescriptor(fn, 1, 2, null);
     expect(result).toBe(undefined);
-    // TODO: This warning is temporarily disabled
-    expect(console.warn.argsForCall.length).toBe(0);
-    // expect(console.warn.argsForCall[0][0]).toContain(
-    //   'This JSX uses a plain function.'
-    // );
+    expect(console.warn.argsForCall.length).toBe(1);
+    expect(console.warn.argsForCall[0][0]).toContain(
+      'This JSX uses a plain function.'
+    );
   });