Do not suppress implicit constructors that need a super call.

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 6c1ba46e5897bfc3447955762756f777dbff9bb3..27432968b2178b751f8e78f047a369b9838c3011 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -682,10 +682,10 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     assert(_hasUnnamedConstructor(node.element) == fields.isNotEmpty);
 
     // If we don't have a method body, skip this.
-    if (fields.isEmpty) return null;
+    var superCall = _superConstructorCall(node);
+    if (fields.isEmpty && superCall == null) return null;
 
     dynamic body = _initializeFields(fields);
-    var superCall = _superConstructorCall(node);
     if (superCall != null) body = [[body, superCall]];
     return new JS.Method(
         _propertyName(name), js.call('function() { #; }', body));