Fix neg tests

Some neg tests now return more errors since we do stricter checking that
comparisons with null make sense. There's an unfortunate interaction with
`errorTerm` in Parser which returns a `null` literal as a tree when the
parser gets confused. That null literal now causes follow on errors in some
cases.

I believe the right thing to do would be to let `errorTerm` return a tree
with an `ErrorType`. I tried that, and it would remove again all the added
errors in this commit. Unfortunately, it breaks SignatureHelpTest in a way
I cannot fix.

Author: odersky

tests/neg/errpos.scala

@@ -10,7 +10,7 @@ object Test {
   val b = type // error: expression expected (on "type")
 
   1 match {
-    case            // error: pattern expected
+    case            // error: pattern expected // error: cannot compare with Null
     case 2 => ""
   }
 }

tests/neg/i1846.scala

@@ -3,17 +3,17 @@ object Test {
     val x = 42
     val Y = "42"
 
-    x match { case { 42 }           => () } // error
-    x match { case { 42.toString }  => () } // error
-    x match { case { 42 }.toString  => () } // error
+    x match { case { 42 }           => () } // error // error
+    x match { case { 42.toString }  => () } // error // error
+    x match { case { 42 }.toString  => () } // error // error
     x match { case "42".toInt       => () } // error
-    x match { case { "42".toInt }   => () } // error
-    x match { case { "42" }.toInt   => () } // error
-    x match { case { "42".toInt }   => () } // error
+    x match { case { "42".toInt }   => () } // error // error
+    x match { case { "42" }.toInt   => () } // error // error
+    x match { case { "42".toInt }   => () } // error // error
     x match { case Y                => () } // error
-    x match { case { Y.toInt }      => () } // error
-    x match { case { Y }.toInt      => () } // error
-    x match { case { Y }.toString   => () } // error
-    x match { case { Y.toString }   => () } // error
+    x match { case { Y.toInt }      => () } // error // error
+    x match { case { Y }.toInt      => () } // error // error
+    x match { case { Y }.toString   => () } // error // error
+    x match { case { Y.toString }   => () } // error // error
   }
 }

tests/neg/i3812.scala

@@ -3,11 +3,11 @@ object Test {
     val x = 42
     val Y = "42"
 
-    x match { case { 42 }           => () } // error
-    x match { case { "42".toInt }   => () } // error
-    x match { case { "42" }.toInt   => () } // error
-    x match { case { "42".toInt }   => () } // error
-    x match { case { Y.toInt }      => () } // error
-    x match { case { Y }.toInt      => () } // error
+    x match { case { 42 }           => () } // error // error
+    x match { case { "42".toInt }   => () } // error // error
+    x match { case { "42" }.toInt   => () } // error // error
+    x match { case { "42".toInt }   => () } // error // error
+    x match { case { Y.toInt }      => () } // error // error
+    x match { case { Y }.toInt      => () } // error // error
   }
 }