Generate code that throws an exception on unimplemented features.

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 d2b10ed5689b46c0c1e3dc0c4956c9695b209ec1..81807b501d30a56fc1c61ab8a2df56852f1f4da7 100644
--- a/dart/frog/leg/resolver.dart
+++ b/dart/frog/leg/resolver.dart
@@ -1125,7 +1125,7 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
   }
 
   visitLiteralMap(LiteralMap node) {
-    unimplemented(node, 'map');
+    warning(node, MessageKind.GENERIC, ['map literals are not implemented']);
   }
 
   visitLiteralMapEntry(LiteralMapEntry node) {
@@ -1137,7 +1137,7 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
   }
 
   visitSwitchStatement(SwitchStatement node) {
-    unimplemented(node, 'switch');
+    warning(node, MessageKind.GENERIC, ['switch statement is not implemented']);
   }
 
   visitTryStatement(TryStatement node) {