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

pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index 19c8a12b08969100bda903ca6b5e9f36760f13fe..99ce5b7c42f2d51f6f5627ad12cd360043d50598 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -350,9 +350,10 @@ class Builder implements cps_ir.Visitor<Node> {
   }
 
   Statement visitInvokeMethod(cps_ir.InvokeMethod node) {
-    Expression invoke = new InvokeMethod(getVariableUse(node.receiver),
-                                         node.selector,
-                                         translateArguments(node.arguments));
+    InvokeMethod invoke = new InvokeMethod(getVariableUse(node.receiver),
+                                           node.selector,
+                                           translateArguments(node.arguments));
+    invoke.receiverIsNotNull = node.receiverIsNotNull;
     return continueWithExpression(node.continuation, invoke);
   }