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

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 1f5fea0e4ab43f379a60c2e3fe484ec279471075..691e206adfad569c27630d3e1cb4b29f5f85bf1f 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -347,8 +347,17 @@ class StatementRewriter extends Transformer implements Pass {
   }
 
   Expression visitInvokeMethod(InvokeMethod node) {
-    _rewriteList(node.arguments);
-    node.receiver = visitExpression(node.receiver);
+    if (node.receiverIsNotNull) {
+      _rewriteList(node.arguments);
+      node.receiver = visitExpression(node.receiver);
+    } else {
+      // Impure expressions cannot be propagated across the method lookup,
+      // because it throws when the receiver is null.
+      inEmptyEnvironment(() {
+        _rewriteList(node.arguments);
+      });
+      node.receiver = visitExpression(node.receiver);
+    }
     return node;
   }