dart2js cps: Check strictness flag in redundant join eliminator.

pkg/compiler/lib/src/cps_ir/optimizers.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 dart2js.cps_ir.optimizers;
 
 import 'cps_ir_nodes.dart';
 import '../constants/values.dart';
 
 export 'type_propagation.dart' show TypePropagator;
@@ skipping 19 matching lines @@
 
 /// Returns true if [value] is false, null, 0, -0, NaN, or the empty string.
 bool isFalsyConstant(ConstantValue value) {
   return value.isFalse ||
       value.isNull  ||
       value.isZero ||
       value.isMinusZero ||
       value.isNaN ||
       value is StringConstantValue && value.primitiveValue.isEmpty;
 }
+
+/// Returns true if [value] satisfies a branching condition with the
+/// given strictness.
+///
+/// For non-strict, this is the opposite of [isFalsyConstant].
+bool isTruthyConstant(ConstantValue value, {bool strict: false}) {
+  return strict ? value.isTrue : !isFalsyConstant(value);
+}