dart2js cps: Check strictness flag in redundant join eliminator.

pkg/compiler/lib/src/cps_ir/optimizers.dart

Index: pkg/compiler/lib/src/cps_ir/optimizers.dart
diff --git a/pkg/compiler/lib/src/cps_ir/optimizers.dart b/pkg/compiler/lib/src/cps_ir/optimizers.dart
index d10823adbd9be674488fca1b8cb1b29c12aa1d13..1ffe52699bae92acd8850d4a7eac9f2c2ce55b8f 100644
--- a/pkg/compiler/lib/src/cps_ir/optimizers.dart
+++ b/pkg/compiler/lib/src/cps_ir/optimizers.dart
@@ -37,3 +37,11 @@ bool isFalsyConstant(ConstantValue value) {
       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);
+}