Only warn that const expressions are not implemented.

dart/frog/leg/resolver.dart

Index: dart/frog/leg/resolver.dart
diff --git a/dart/frog/leg/resolver.dart b/dart/frog/leg/resolver.dart
index 1b68d4fcd3d7ce2ca572fa2f95a86497b569b1ef..f5ae8c7c2e92d020b866425b9f2ac21fa19f5610 100644
--- a/dart/frog/leg/resolver.dart
+++ b/dart/frog/leg/resolver.dart
@@ -852,7 +852,10 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
   }
 
   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(
@@ -915,7 +918,10 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
   }
 
   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);
   }