dart2js cps: Narrow the receiver mask on InvokeMethod.

pkg/compiler/lib/src/cps_ir/type_mask_system.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.type_mask_system;
 
 import '../common/names.dart' show Selectors, Identifiers;
 import '../compiler.dart' as dart2js show Compiler;
 import '../constants/values.dart';
 import '../dart_types.dart' as types;
@@ skipping 115 matching lines @@
   }
 
   TypeMask getFieldType(FieldElement field) {
     return inferrer.getGuaranteedTypeOfElement(field);
   }
 
   TypeMask join(TypeMask a, TypeMask b) {
     return a.union(b, classWorld);
   }
 
+  TypeMask intersection(TypeMask a, TypeMask b) {
+    if (a == null) return b;
+    if (b == null) return a;
+    return a.intersection(b, classWorld);
+  }
+
   TypeMask getTypeOf(ConstantValue constant) {
     return computeTypeMask(inferrer.compiler, constant);
   }
 
   // Returns the constant value if a TypeMask represents a single value.
   // Returns `null` if [mask] is not a constant.
   ConstantValue getConstantOf(TypeMask mask) {
     if (!mask.isValue) return null;
     if (mask.isNullable) return null;  // e.g. 'true or null'.
     ValueTypeMask valueMask = mask;
@@ skipping 237 matching lines @@
 
   /// The length of something of [type], or `null` if unknown.
   int getContainerLength(TypeMask type) {
     if (type is ContainerTypeMask) {
       return type.length;
     } else {
       return null;
     }
   }
 }