dart2js cps: Do not propagate expressions into foreign code.

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/values.dart' as values;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
 import '../io/source_information.dart' show SourceInformation;
@@ skipping 101 matching lines @@
   /// Number of places where this variable occurs in a [VariableUse].
   int readCount = 0;
 
   /// Number of places where this variable occurs as:
   /// - left-hand of an [Assign]
   /// - left-hand of a [FunctionDeclaration]
   /// - parameter in a [FunctionDefinition]
   /// - catch parameter in a [Try]
   int writeCount = 0;
 
-  /// True if a nested function reads or writes this variable.
-  ///
-  /// Always false in JS-mode because closure conversion eliminated nested
-  /// functions.
+  /// True if an inner JS function might access this variable through a
+  /// [ForeignCode] node.
   bool isCaptured = false;
 
   Variable(this.host, this.element) {
     assert(host != null);
   }
 
   String toString() => element == null ? 'Variable' : element.toString();
 }
 
 /// Read the value of a variable.
@@ skipping 1462 matching lines @@
 
   /// Number of uses of the current fallthrough target.
   int get useCount => _stack.last.useCount;
 
   /// Indicate that a statement will fall through to the current fallthrough
   /// target.
   void use() {
     ++_stack.last.useCount;
   }
 }