Merge two static_final_field tests and rewrite a negative test to a multi-test

tests/language/scope_variable_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.
 
 import "package:expect/expect.dart";
 
 void testSimpleScope() {
   {
     var a = "Test";
     int b = 1;
   }
   {
     var c;
     int d;
     Expect.isNull(c);
     Expect.isNull(d);
   }
 }
 
 void testShadowingScope() {
   var a = "Test";
   {
     var a;
-    Expect.equals(true, a == null);
+    Expect.isNull(a);
     a = "a";
-    Expect.equals(true, a == "a");
+    Expect.equals(a, "a");
   }
-  Expect.equals(true, a == "Test");
+  Expect.equals(a, "Test");
 }
 
 int testShadowingAfterUse() {
   var a = 1;
   {
     var b = 2;
     var c = a;  // Use of 'a' prior to its shadow declaration below.
     var d = b + c;
     // Shadow declaration of 'a'.
     var a = 5;  /// 01: compile-time error
     return d + a;
   }
 }
 
 main() {
   testSimpleScope();
   testShadowingScope();
   testShadowingAfterUse();
 }