Add structural equality to ConstantExpression.

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

Index: pkg/compiler/lib/src/js_backend/codegen/unsugar.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/unsugar.dart b/pkg/compiler/lib/src/js_backend/codegen/unsugar.dart
index 5560df5660e0f2f7e6c1806b82ee3fa5892420c3..f439e95de4fdab81de1f84f64b2903319cbc4feb 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/unsugar.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/unsugar.dart
@@ -37,7 +37,8 @@ class UnsugarVisitor extends RecursiveVisitor {
 
   Constant get trueConstant {
     return new Constant(
-        new PrimitiveConstantExpression(
+        new BoolConstantExpression(
+            true,
             new TrueConstantValue()));
   }
 
@@ -135,7 +136,7 @@ class UnsugarVisitor extends RecursiveVisitor {
 
   Primitive makeNull() {
     NullConstantValue nullConst = new NullConstantValue();
-    return new Constant(new PrimitiveConstantExpression(nullConst));
+    return new Constant(new NullConstantExpression(nullConst));
   }
 
   processInvokeMethodDirectly(InvokeMethodDirectly node) {