dart2js cps: Do not propagate impure expressions across null receiver.

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

 // Copyright (c) 2013, 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.ir_nodes;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart' as values show ConstantValue;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
 import '../io/source_information.dart' show SourceInformation;
@@ skipping 251 matching lines @@
 /// Discussion:
 /// If the [selector] is a [TypedSelector], the type information contained
 /// there is used by optimization passes. This is likely to change.
 class InvokeMethod extends Expression implements Invoke {
   Reference<Primitive> receiver;
   Selector selector;
   final List<Reference<Primitive>> arguments;
   final Reference<Continuation> continuation;
   final SourceInformation sourceInformation;
 
+  /// If true, it is known that the receiver cannot be `null`.
+  ///
+  /// This field is `null` until initialized by optimization phases.
+  bool receiverIsNotNull;
+
   InvokeMethod(Primitive receiver,
                this.selector,
                List<Primitive> arguments,
                Continuation continuation,
                {this.sourceInformation})
       : this.receiver = new Reference<Primitive>(receiver),
         this.arguments = _referenceList(arguments),
         this.continuation = new Reference<Continuation>(continuation);
 
   accept(Visitor visitor) => visitor.visitInvokeMethod(this);
@@ skipping 883 matching lines @@
     processNonTailThrow(node);
     processReference(node.value);
   }
 
   processCreateInvocationMirror(CreateInvocationMirror node) {}
   visitCreateInvocationMirror(CreateInvocationMirror node) {
     processCreateInvocationMirror(node);
     node.arguments.forEach(processReference);
   }
 }