Fix const unparse. Example:

lib/compiler/implementation/tree/unparser.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class Unparser implements Visitor {
   StringBuffer sb;
 
   Unparser();
 
   String unparse(Node node) {
@@ skipping 133 matching lines @@
   }
 
   visitNewExpression(NewExpression node) {
     // TODO(ahe): handle 'const'.
     add(node.newToken.value);
     sb.add(' ');
     visit(node.send);
   }
 
   visitLiteralList(LiteralList node) {
-    // TODO(ahe): handle 'const'.
+    if (node.constKeyword !== null) {
+      add(node.constKeyword.value);
+    }
     if (node.type !== null) {
       sb.add('<');
       visit(node.type);
       sb.add('>');
     }
     sb.add(' ');
     visit(node.elements);
   }
 
   visitModifiers(Modifiers node) => node.visitChildren(this);
@@ skipping 196 matching lines @@
     visit(node.identifier);
     add(node.colonToken.value);
    }
 
   visitLabeledStatement(LabeledStatement node) {
     visit(node.labels);
     visit(node.statement);
   }
 
   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);
   }
 
   visitLiteralMapEntry(LiteralMapEntry node) {
     visit(node.key);
     add(node.colonToken.value);
     sb.add(' ');
     visit(node.value);
   }
@@ skipping 71 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }