dart2js cps: Store the TypeMask for each primitive in a field.

pkg/compiler/lib/src/cps_ir/share_interceptors.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.
 
 library dart2js.cps_ir.share_interceptors;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
+import 'type_mask_system.dart';
+import '../elements/elements.dart';
 import '../constants/values.dart';
 
-/// Merges calls to `getInterceptor` when one call dominates the other.
+/// Merges calls to `getInterceptor` when one call is in scope of the other.
+///
+/// Also replaces `getInterceptor` calls with an interceptor constant when
+/// the result is known statically, and there is no interceptor already in
+/// scope.
 /// 
 /// Should run after [LoopInvariantCodeMotion] so interceptors lifted out from
 /// loops can be merged.
 class ShareInterceptors extends RecursiveVisitor implements Pass {
   String get passName => 'Share interceptors';
 
   final Map<Primitive, Primitive> interceptorFor =
       <Primitive, Primitive>{};
 
   final Map<ConstantValue, Primitive> sharedConstantFor =
@@ skipping 61 matching lines @@
     return node.body;
   }
 
   Expression getEnclosingExpression(Node node) {
     while (node is! Expression) {
       node = node.parent;
     }
     return node;
   }
 }