Warnings for instantiating an abstract type.

frog/member.dart

Index: frog/member.dart
diff --git a/frog/member.dart b/frog/member.dart
index f2481da856da483d5dbecefc5793b10d64831e46..22824d4c326aefe0d1bb378bf4dae678514e7669 100644
--- a/frog/member.dart
+++ b/frog/member.dart
@@ -1005,6 +1005,32 @@ class MethodMember extends Member {
           : '${declaringType.jsname}.call($argsString)';
       return new Value(target.type, code, node.span);
     } else {
+      if (declaringType.isAbstract) {
+        // Warn at the constructor location
+        world.warning('Cannot instantiate abstract class ${declaringType.name}',
+            node.span);
+
+        // Now make a message explaining why the type is abstract.
+
+        // Get abstract members grouped by type
+        var members = groupBy(declaringType.inheritedMembers.getValues().filter(
+            (m) => m.isAbstract), (m) => m.declaringType);
+
+        var msg = new StringBuffer('Type ${declaringType.name} is abstract '
+            + 'because it does not implement the following members');
+
+        var types = members.getKeys();
+        types.sort((x, y) => x.name.compareTo(y.name));
+        for (var type in types) {
+          if (type != types[0]) msg.add(';');
+          msg.add(' from type ${type.name}: ');
+          msg.add(Strings.join(map(members[type], (m) => m.name), ', '));
+        }
+
+        // Warn at the type's location
+        world.warning(msg.toString(), declaringType.span);
+      }
+
       var code = (constructorName != '')
           ? 'new ${declaringType.jsname}.${constructorName}\$ctor($argsString)'
           : 'new ${declaringType.jsname}($argsString)';