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

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

Index: pkg/analyzer/lib/src/task/strong/info.dart
diff --git a/pkg/analyzer/lib/src/task/strong/info.dart b/pkg/analyzer/lib/src/task/strong/info.dart
index 408c05dae2033756d6231e3a1cf1a6b0b6f46bab..ec8626fd7e1ff93e6ffd2d8ff6c85277a6bb167f 100644
--- a/pkg/analyzer/lib/src/task/strong/info.dart
+++ b/pkg/analyzer/lib/src/task/strong/info.dart
@@ -92,12 +92,6 @@ abstract class DownCast extends CoercionInfo {
     // 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