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

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

 // Copyright (c) 2014, 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 tree_ir.optimization.statement_rewriter;
 
 import 'optimization.dart' show Pass;
 import '../tree_ir_nodes.dart';
 
 /**
@@ skipping 329 matching lines @@
       nodes[i] = visitExpression(nodes[i]);
     }
   }
 
   Expression visitInvokeStatic(InvokeStatic node) {
     _rewriteList(node.arguments);
     return node;
   }
 
   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;
   }
 
   Expression visitInvokeMethodDirectly(InvokeMethodDirectly node) {
     _rewriteList(node.arguments);
     node.receiver = visitExpression(node.receiver);
     return node;
   }
 
   Expression visitInvokeConstructor(InvokeConstructor node) {
@@ skipping 541 matching lines @@
 }
 
 /// Result of combining two expressions that do not affect reference counting.
 class GenericCombinedExpressions implements CombinedExpressions {
   Expression combined;
 
   GenericCombinedExpressions(this.combined);
 
   void uncombine() {}
 }