dart2js cps_ir: Use interceptors for is-checks.

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 a84d28d4ba5cd333e83868f839722da0da625407..426f56866f49bed10891a1c825321c696d5c602a 100644
--- a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
@@ -42,6 +42,7 @@ class TypeMaskSystem {
   TypeMask get extendableNativeListType => backend.extendableArrayType;
 
   TypeMask numStringBoolType;
+  TypeMask interceptorType;
 
   ClassElement get jsNullClass => backend.jsNullClass;
 
@@ -63,6 +64,9 @@ class TypeMaskSystem {
     numStringBoolType =
         new TypeMask.unionOf(<TypeMask>[anyNum, anyString, anyBool],
             classWorld);
+
+    interceptorType = new TypeMask.nonNullSubtype(
+        backend.jsInterceptorClass, classWorld);
   }
 
   bool methodUsesReceiverArgument(FunctionElement function) {
@@ -199,6 +203,9 @@ class TypeMaskSystem {
     return t.satisfies(backend.jsIntClass, classWorld);
   }
 
+  // TODO(sra): Find a better name.  'NativeList' is a bad name because there
+  // are many native classes in dart:html that implement List but are not (and
+  // should not be) included in this predicate.
   bool isDefinitelyNativeList(TypeMask t, {bool allowNull: false}) {
     if (!allowNull && t.isNullable) return false;
     return t.nonNullable().satisfies(backend.jsArrayClass, classWorld);
@@ -225,6 +232,11 @@ class TypeMaskSystem {
     return t.nonNullable().satisfies(backend.jsIndexableClass, classWorld);
   }
 
+  bool isDefinitelyNotIntercepted(TypeMask t, {bool allowNull: false}) {
+    if (!allowNull && t.isNullable) return false;
+    return areDisjoint(t, interceptorType);
+  }
+
   bool areDisjoint(TypeMask leftType, TypeMask rightType) {
     TypeMask intersection = leftType.intersection(rightType, classWorld);
     return intersection.isEmpty && !intersection.isNullable;