Report errors when factory redirection is used in non-factory methods.

dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart

Index: dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart b/dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart
index 8c2225a0c34fc77a930af834283ab961650332a2..d80c72c8c7a47c1904a4f26c3c235c8f5cbfc8fc 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart
@@ -2012,13 +2012,22 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
   }
 
   void handleRedirectingFactoryBody(Return node) {
+    if (!enclosingElement.isFactoryConstructor()) {
+      compiler.reportMessage(
+          compiler.spanFromNode(node),
+          MessageKind.FACTORY_REDIRECTION_IN_NON_FACTORY.error([]),
+          Diagnostic.ERROR);
+      compiler.reportMessage(
+          compiler.spanFromElement(enclosingElement),
+          MessageKind.MISSING_FACTORY_KEYWORD.error([]),
+          Diagnostic.INFO);
+    }
     Element redirectionTarget = resolveRedirectingFactory(node);
     var type = mapping.getType(node.expression);
     if (type is InterfaceType && !type.isRaw) {
       unimplemented(node.expression, 'type arguments on redirecting factory');
     }
     useElement(node.expression, redirectionTarget);
-    assert(invariant(node, enclosingElement.isFactoryConstructor()));
     FunctionElement constructor = enclosingElement;
     if (constructor.modifiers.isConst() &&
         !redirectionTarget.modifiers.isConst()) {