insert type assertions when narrowing.

R=r
OCL=24349
CL=24913
diff --git a/test/interface5.go b/test/interface5.go
index 2e273dd..86036a7 100644
--- a/test/interface5.go
+++ b/test/interface5.go
@@ -12,10 +12,21 @@
 type I interface { M() }
 var i I
 
+type I2 interface { M(); N(); }
+var i2 I2;
+
+var e interface { };
+
 func main() {
+	e = t;	// ok
+	t = e;	// ERROR "need explicit"
+
 	// neither of these can work,
 	// because i has an extra method
 	// that t does not, so i cannot contain a t.
-	i = t;	// ERROR "missing|incompatible"
-	t = i;	// ERROR "missing|incompatible"
+	i = t;	// ERROR "missing|incompatible|is not"
+	t = i;	// ERROR "missing|incompatible|is not"
+
+	i = i2;	// ok
+	i2 = i;	// ERROR "need explicit"
 }