Only warn that const expressions are not implemented.

dart/frog/leg/resolver.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.
 
 interface TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
 }
 
 class TreeElementMapping implements TreeElements {
@@ skipping 834 matching lines @@
   visitWhile(While node) {
     visit(node.condition);
     visitLoopBodyIn(node, node.body, new BlockScope(context));
   }
 
   visitParenthesizedExpression(ParenthesizedExpression node) {
     visit(node.expression);
   }
 
   visitNewExpression(NewExpression node) {
-    if (node.isConst()) cancel(node, 'const expressions are not implemented');
+    if (node.isConst()) {
+      warning(node, MessageKind.GENERIC,
+              ['const expressions are not implemented']);
+    }
     Node selector = node.send.selector;
     if (selector.asTypeAnnotation() === null) {
       cancel(
           node, 'named constructors with type arguments are not implemented');
     }
 
     FunctionElement constructor = resolveConstructor(node);
     handleArguments(node.send);
     if (constructor === null) return null;
     // TODO(karlklose): handle optional arguments.
@@ skipping 42 matching lines @@
     typeRequired = old;
     return cls;
   }
 
   visitModifiers(Modifiers node) {
     // TODO(ngeoffray): Implement this.
     unimplemented(node, 'modifiers');
   }
 
   visitLiteralList(LiteralList node) {
-    if (node.isConst()) cancel(node, 'const literal lists are not implemented');
+    if (node.isConst()) {
+      warning(node, MessageKind.GENERIC,
+              ['const literal lists are not implemented']);
+    }
     visit(node.elements);
   }
 
   visitConditional(Conditional node) {
     node.visitChildren(this);
   }
 
   visitStringInterpolation(StringInterpolation node) {
     node.visitChildren(this);
   }
@@ skipping 420 matching lines @@
 class TopScope extends Scope {
   LibraryElement get library() => element;
 
   TopScope(LibraryElement library) : super(null, library);
   Element lookup(SourceString name) => library.find(name);
 
   Element add(Element element) {
     throw "Cannot add an element in the top scope";
   }
 }