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

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.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_nodes;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart' as values;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
@@ skipping 174 matching lines @@
 
   accept(ExpressionVisitor visitor) => visitor.visitInvokeStatic(this);
   accept1(ExpressionVisitor1 visitor, arg) {
     return visitor.visitInvokeStatic(this, arg);
   }
 }
 
 /**
  * A call to a method, operator, getter, setter or index getter/setter.
  *
- * In contrast to the CPS-based IR, the receiver and arguments can be
- * arbitrary expressions.
+ * If [receiver] is `null`, an error is thrown before the arguments are
+ * evaluated. This corresponds to the JS evaluation order.
  */
 class InvokeMethod extends Expression implements Invoke {
   Expression receiver;
   final Selector selector;
   final List<Expression> arguments;
 
+  /// If true, it is known that the receiver cannot be `null`.
+  bool receiverIsNotNull = false;
+
   InvokeMethod(this.receiver, this.selector, this.arguments) {
     assert(receiver != null);
   }
 
   accept(ExpressionVisitor visitor) => visitor.visitInvokeMethod(this);
   accept1(ExpressionVisitor1 visitor, arg) {
     return visitor.visitInvokeMethod(this, arg);
   }
 }
 
 /// Invoke [target] on [receiver], bypassing ordinary dispatch semantics.
+///
+/// Since the [receiver] is not used for method lookup, it may be `null`
+/// without an error being thrown.
 class InvokeMethodDirectly extends Expression implements Invoke {
   Expression receiver;
   final Element target;
   final Selector selector;
   final List<Expression> arguments;
 
   InvokeMethodDirectly(this.receiver, this.target, this.selector,
       this.arguments);
 
   accept(ExpressionVisitor visitor) => visitor.visitInvokeMethodDirectly(this);
@@ skipping 967 matching lines @@
   visitTypeExpression(TypeExpression node) {
     _replaceExpressions(node.arguments);
     return node;
   }
 
   visitCreateInvocationMirror(CreateInvocationMirror node) {
     _replaceExpressions(node.arguments);
     return node;
   }
 }