Remove dead code in strong mode's warnings for null literals

pkg/analyzer/lib/src/task/strong/info.dart

 // Copyright (c) 2015, 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.
 
 /// Defines static information collected by the type checker and used later by
 /// emitters to generate code.
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.info;
 
@@ skipping 74 matching lines @@
   String get message => 'Unsound implicit cast from {0} to {1}';
 
   /// Factory to create correct DownCast variant.
   static StaticInfo create(StrongTypeSystemImpl rules, Expression expression,
       DartType fromType, DartType toType) {
     // toT <:_R fromT => to <: fromT
     // NB: classes with call methods are subtypes of function
     // types, but the function type is not assignable to the class
     assert(toType.isSubtypeOf(fromType) || fromType.isAssignableTo(toType));
 
-    // Handle null call specially.
-    if (expression is NullLiteral) {
-      // TODO(vsm): Create a NullCast for this once we revisit nonnullability.
-      return new DownCastImplicit(rules, expression, fromType, toType);
-    }
-
     // Inference "casts":
     if (expression is Literal || expression is FunctionExpression) {
       // fromT should be an exact type - this will almost certainly fail at
       // runtime.
       return new StaticTypeError(rules, expression, toType);
     }
 
     if (expression is InstanceCreationExpression) {
       ConstructorElement e = expression.staticElement;
       if (e == null || !e.isFactory) {
@@ skipping 427 matching lines @@
         super(expression);
 
   @override
   List<Object> get arguments => [node, baseType, expectedType];
   @override
   String get message => 'Type check failed: {0} ({1}) is not of type {2}';
 
   @override
   String get name => 'STRONG_MODE_STATIC_TYPE_ERROR';
 }