dart2js: improve ssa utilization of bool value types

pkg/compiler/lib/src/types/value_type_mask.dart

Index: pkg/compiler/lib/src/types/value_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/value_type_mask.dart b/pkg/compiler/lib/src/types/value_type_mask.dart
index 737be5e3a513e0e65e71bfbdddc8a275208e2a29..72c5af28e7a4ca1192da5fb7a19b21c71cd40e7f 100644
--- a/pkg/compiler/lib/src/types/value_type_mask.dart
+++ b/pkg/compiler/lib/src/types/value_type_mask.dart
@@ -6,7 +6,7 @@ part of types;
 
 class ValueTypeMask extends ForwardingTypeMask {
   final TypeMask forwardTo;
-  final value;
+  final PrimitiveConstantValue value;
 
   ValueTypeMask(this.forwardTo, this.value);
 
@@ -44,6 +44,6 @@ class ValueTypeMask extends ForwardingTypeMask {
   }
 
   String toString() {
-    return 'Value mask: [$value] type: $forwardTo';
+    return 'Value mask: [${value.unparse()}] type: $forwardTo';
   }
 }