Fix const unparse. Example:

lib/compiler/implementation/tree/unparser.dart

Index: lib/compiler/implementation/tree/unparser.dart
diff --git a/lib/compiler/implementation/tree/unparser.dart b/lib/compiler/implementation/tree/unparser.dart
index 160890e179a476d2331e605ad765b75ac94161aa..fc1d85fdd750e6d6fd46f5d776a0c661d6b44b50 100644
--- a/lib/compiler/implementation/tree/unparser.dart
+++ b/lib/compiler/implementation/tree/unparser.dart
@@ -151,7 +151,9 @@ class Unparser implements Visitor {
   }
 
   visitLiteralList(LiteralList node) {
-    // TODO(ahe): handle 'const'.
+    if (node.constKeyword !== null) {
+      add(node.constKeyword.value);
+    }
     if (node.type !== null) {
       sb.add('<');
       visit(node.type);
@@ -368,7 +370,9 @@ class Unparser implements Visitor {
   }
 
   visitLiteralMap(LiteralMap node) {
-    // TODO(ahe): handle 'const'.
+    if (node.constKeyword !== null) {
+      add(node.constKeyword.value);
+    }
     if (node.typeArguments !== null) visit(node.typeArguments);
     visit(node.entries);
   }