test that cell magic preempts other transformers

IPython/core/tests/test_inputsplitter.py

@@ -446,6 +446,22 b' class IPythonInputTestCase(InputSplitterTestCase):'
                 out = isp.transform_cell(raw)
                 # Match ignoring trailing whitespace
                 self.assertEqual(out.rstrip(), out_t.rstrip())
+    
+    def test_cellmagic_preempt(self):
+        isp = self.isp
+        for raw, name, line, cell in [
+            ("%%cellm a\nIn[1]:", u'cellm', u'a', u'In[1]:'),
+            ("%%cellm \n...:", u'cellm', u'', u'...:'),
+            ("%%cellm \nline1\nline2", u'cellm', u'', u'line1\nline2'),
+            ("%%cellm \nline1\\\\\nline2", u'cellm', u'', u'line1\\\\\nline2'),
+        ]:
+            expected = "get_ipython().run_cell_magic(%r, %r, %r)" % (
+                name, line, cell
+            )
+            out = isp.transform_cell(raw)
+            self.assertEqual(out.rstrip(), expected.rstrip())
+        
+        
 
 #-----------------------------------------------------------------------------
 # Main - use as a script, mostly for developer experiments