Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart

 library dart2js.cps_ir.backward_null_check_remover;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
-import '../common/names.dart';
-import '../universe/selector.dart';
 import 'type_mask_system.dart';
 import 'cps_fragment.dart';
 
 /// Removes null checks that are follwed by another instruction that will
 /// perform the same check.
 ///
 /// For example:
 ///
 ///     x.toString; // NullCheck instruction
 ///     print(x.length);
@@ skipping 102 matching lines @@
   void visitLetHandler(LetHandler node) {
     nullCheckedValue = null;
   }
 
   visitInvokeContinuation(InvokeContinuation node) {
     if (!node.isRecursive) {
       nullCheckedValue = nullCheckedValueAt[node.continuation];
     }
   }
 }