Don't transform interceptors into dynamic invocations for null.

lib/compiler/implementation/ssa/nodes.dart

 // Copyright (c) 2012, 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.
 
 interface HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
   R visitBitXor(HBitXor node);
@@ skipping 752 matching lines @@
   bool isExtendableArray() => propagatedType.isExtendableArray();
   bool isBoolean() => propagatedType.isBoolean();
   bool isInteger() => propagatedType.isInteger();
   bool isDouble() => propagatedType.isDouble();
   bool isNumber() => propagatedType.isNumber();
   bool isString() => propagatedType.isString();
   bool isTypeUnknown() => propagatedType.isUnknown();
   bool isIndexablePrimitive() => propagatedType.isIndexablePrimitive();
   bool isPrimitive() => propagatedType.isPrimitive();
   bool canBePrimitive() => propagatedType.canBePrimitive();
+  bool canBeNull() => propagatedType.canBeNull();
 
   /**
    * This is the type the instruction is guaranteed to have. It does not
    * take any propagation into account.
    */
   HType guaranteedType = HType.UNKNOWN;
   bool hasGuaranteedType() => !guaranteedType.isUnknown();
 
   /**
    * The [propagatedType] is the type the instruction is assumed to have.
@@ skipping 1835 matching lines @@
   HBasicBlock get start() => expression.start;
   HBasicBlock get end() {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty());
     return statements.last().end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }