dart2js cps: Narrow the receiver mask on InvokeMethod.

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

Index: pkg/compiler/lib/src/cps_ir/type_mask_system.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
index ba44ed1c26068069ff6a6f6fa747d90ff988e5c5..ec9cb433f99172f694275a48340d690c9cbfc5c7 100644
--- a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
@@ -133,6 +133,12 @@ class TypeMaskSystem {
     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);
   }