Made removeAt errors consistent between dart2js and VM.

tests/corelib/list_removeat_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 void main() {
   // Normal modifiable list.
   var l1 = [0, 1, 2, 3, 4];
 
+  bool checkedMode = false;
+  assert(checkedMode = true);
+
   // Index must be integer and in range.
   Expect.throws(() { l1.removeAt(-1); },
                 (e) => e is IndexOutOfRangeException,
                 "negative");
   Expect.throws(() { l1.removeAt(5); },
                 (e) => e is IndexOutOfRangeException,
                 "too large");
+  Expect.throws(() { l1.removeAt(null); },
+                (e) => e is IllegalArgumentException,
+                "too large");
   Expect.throws(() { l1.removeAt("1"); },
-                (e) => e is IllegalArgumentException,
+                (e) => (checkedMode ? e is TypeError
+                                    : e is IllegalArgumentException),
                 "string");
   Expect.throws(() { l1.removeAt(1.5); },
-                (e) => e is IllegalArgumentException,
+                (e) => (checkedMode ? e is TypeError
+                                    : e is IllegalArgumentException),
                 "double");
 
   Expect.equals(2, l1.removeAt(2), "l1-remove2");
   Expect.equals(1, l1[1], "l1-1[1]");
 
   Expect.equals(3, l1[2], "l1-1[2]");
   Expect.equals(4, l1[3], "l1-1[3]");
   Expect.equals(4, l1.length, "length-1");
 
   Expect.equals(0, l1.removeAt(0), "l1-remove0");
@@ skipping 14 matching lines @@
   Expect.throws(() { l3.removeAt(2); },
                 (e) => e is UnsupportedOperationException,
                 "unmodifiable");
 
   // Empty list is not special.
   var l4 = [];
   Expect.throws(() { l4.removeAt(0); },
                 (e) => e is IndexOutOfRangeException,
                 "empty");
 }