Implement implicit constructors in mixin applications.

sdk/lib/_internal/compiler/implementation/enqueue.dart

Index: sdk/lib/_internal/compiler/implementation/enqueue.dart
diff --git a/sdk/lib/_internal/compiler/implementation/enqueue.dart b/sdk/lib/_internal/compiler/implementation/enqueue.dart
index 768ec2f11b2674fdabecf2ff937632a1989cdbea..1278cd0627fcbf7a39a63994e254360737f4b5c9 100644
--- a/sdk/lib/_internal/compiler/implementation/enqueue.dart
+++ b/sdk/lib/_internal/compiler/implementation/enqueue.dart
@@ -59,6 +59,11 @@ abstract class Enqueuer {
     assert(invariant(element, element.isDeclaration));
     if (element.isForeign(compiler)) return;
 
+    if (element.isForwardingConstructor) {
+      addToWorkList(element.targetConstructor, elements);
+      return;
+    }
+
     if (!addElementToWorkList(element, elements)) return;
 
     // Enable runtime type support if we discover a getter called runtimeType.