Further reduce use of Compiler.closedWorld.

pkg/compiler/lib/src/types/masks.dart

Index: pkg/compiler/lib/src/types/masks.dart
diff --git a/pkg/compiler/lib/src/types/masks.dart b/pkg/compiler/lib/src/types/masks.dart
index bee132d3d3f668070122b0707d83c9a72dabac65..f44446ab6abc0717b1b944538bdab7b4affca667 100644
--- a/pkg/compiler/lib/src/types/masks.dart
+++ b/pkg/compiler/lib/src/types/masks.dart
@@ -67,6 +67,7 @@ class CommonMasks {
   TypeMask _fixedArrayType;
   TypeMask _extendableArrayType;
   TypeMask _unmodifiableArrayType;
+  TypeMask _interceptorType;
 
   TypeMask get dynamicType => _dynamicType ??=
       new TypeMask.subclass(closedWorld.coreClasses.objectClass, closedWorld);
@@ -164,6 +165,10 @@ class CommonMasks {
       _unmodifiableArrayType ??= new TypeMask.nonNullExact(
           backendClasses.constListImplementation, closedWorld);
 
+  TypeMask get interceptorType =>
+      _interceptorType ??= new TypeMask.nonNullSubclass(
+          backendClasses.interceptorImplementation, closedWorld);
+
   bool isTypedArray(TypeMask mask) {
     // Just checking for [:TypedData:] is not sufficient, as it is an
     // abstract class any user-defined class can implement. So we also