dart2js cps: Fix some analyzer hints in the CPS backend.

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

 library dart2js.cps_ir.loop_effects;
 
 import 'cps_ir_nodes.dart';
 import 'loop_hierarchy.dart';
-import 'type_mask_system.dart';
 import '../universe/side_effects.dart';
 import '../elements/elements.dart';
 import '../world.dart';
 
 /// Determines which the [SideEffects] that may occur during each loop in
 /// a given function, in addition to whether the loop may change the length
 /// of an indexable object.
 ///
 /// TODO(asgerf): Make length a flag on [SideEffects] for better precision and
 /// so we don't need to special case the length in this class.
@@ skipping 161 matching lines @@
 
   void visitYield(Yield node) {
     addAllSideEffects();
   }
 
   void visitApplyBuiltinMethod(ApplyBuiltinMethod node) {
     currentLoopSideEffects.setChangesIndex();
     currentLoopChangesLength = true; // Push and pop.
   }
 }