Make Invocation.positionalArguments and .namedArguments never be null.

tests/language/invocation_mirror_test.dart

 // Copyright (c) 2012, 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.
 
 import "package:expect/expect.dart";
 
 // Invocation and noSuchMethod testing.
 
 Map<Symbol, dynamic> listToNamedArguments(list) {
   var iterator = list.iterator;
@@ skipping 55 matching lines @@
                   "$name:positional[0]");
     Expect.equals(0, im.namedArguments.length, "$name:#named");
     return;
   }
   Map<Symbol, dynamic> namedArguments = listToNamedArguments(named);
   Expect.isTrue(im.isMethod, "$name:isMethod");
   Expect.isFalse(im.isAccessor, "$name:isAccessor");
   Expect.isFalse(im.isSetter, "$name:isSetter");
   Expect.isFalse(im.isGetter, "$name:isGetter");
 
-  Expect.equals(positional.length, im.positionalArguments.length);
-  for (int i = 0; i < positional.length; i++) {
-    Expect.equals(positional[i], im.positionalArguments[i],
-                  "$name:positional[$i]");
-  }
+  Expect.listEquals(positional, im.positionalArguments);
+
   Expect.equals(namedArguments.length, im.namedArguments.length,
                 "$name:#named");
   namedArguments.forEach((k, v) {
     Expect.isTrue(im.namedArguments.containsKey(k),
                   "$name:?namedArguments[$k]");
     Expect.equals(v, im.namedArguments[k], "$name:namedArguments[$k]");
   });
 }
 
 
@@ skipping 198 matching lines @@
   test(() => o.hashCode = 42);
   test(() => o.hashCode());  // Thrown by int.noSuchMethod.
   test(() => (n.flif)());  // Extracted method has no noSuchMethod.
 }
 
 main() {
   testInvocationMirrors();
   testNoSuchMethodErrors();
   new M().testSuperCalls();
 }