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

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
index d052121e9ce2ce1aec11e740784e965075c61085..ea3f5395e25386ba602ba769ab26737d9b765443 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
@@ -269,6 +269,11 @@ class InvokeMethod extends Expression implements Invoke {
   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,