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

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of resolution;
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(Node node);
@@ skipping 1994 matching lines @@
 
   visitReturn(Return node) {
     if (node.isRedirectingFactoryBody) {
       handleRedirectingFactoryBody(node);
     } else {
       visit(node.expression);
     }
   }
 
   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()) {
       error(node, MessageKind.CONSTRUCTOR_IS_NOT_CONST);
     }
     // TODO(ahe): Check that this doesn't lead to a cycle.  For now,
     // just make sure that the redirection target isn't itself a
     // redirecting factory.
     { // This entire block is temporary code per the above TODO.
       FunctionElement targetImplementation = redirectionTarget.implementation;
@@ skipping 1166 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }