dart2js cps: Refactor and optimize string concatenations.

pkg/compiler/lib/src/js_backend/codegen/unsugar.dart

 library dart2js.unsugar_cps;
 
 import '../../cps_ir/cps_ir_nodes.dart';
 
 import '../../cps_ir/optimizers.dart' show ParentVisitor;
 import '../../constants/expressions.dart';
 import '../../constants/values.dart';
 import '../../elements/elements.dart' show
     ClassElement,
     FieldElement,
@@ skipping 64 matching lines @@
     visit(function);
   }
 
   @override
   visit(Node node) {
     Node result = node.accept(this);
     return result != null ? result : node;
   }
 
   Constant get trueConstant {
-    return new Constant(
-        new BoolConstantExpression(true),
-        new TrueConstantValue());
+    return new Constant(new TrueConstantValue());
   }
 
   Constant get falseConstant {
-    return new Constant(
-        new BoolConstantExpression(false),
-        new FalseConstantValue());
+    return new Constant(new FalseConstantValue());
   }
 
   Constant get nullConstant {
-    return new Constant(
-        new NullConstantExpression(),
-        new NullConstantValue());
+    return new Constant(new NullConstantValue());
   }
 
   void insertLetPrim(Primitive primitive, Expression node) {
     LetPrim let = new LetPrim(primitive);
     InteriorNode parent = node.parent;
     parent.body = let;
     let.body = node;
     node.parent = let;
     let.parent = parent;
   }
@@ skipping 175 matching lines @@
     condition.value.unlink();
     node.trueContinuation.unlink();
     node.falseContinuation.unlink();
     parent.body = newNode;
   }
 
   processInterceptor(Interceptor node) {
     _glue.registerSpecializedGetInterceptor(node.interceptedClasses);
   }
 }