Do not add forwarding constructors to a pricate constructor from a different library.

sdk/lib/_internal/compiler/implementation/ssa/builder.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 ssa;
 
 /// A synthetic local variable only used with the SSA graph.
 ///
 /// For instance used for holding return value of function or the exception of a
 /// try-catch statement.
@@ skipping 1744 matching lines @@
                          List<FunctionElement> constructors,
                          Map<Element, HInstruction> fieldValues) {
     assert(invariant(constructor, constructor.isImplementation));
     if (constructor.isSynthesized) {
       List<HInstruction> arguments = <HInstruction>[];
       HInstruction compileArgument(ParameterElement parameter) {
         return localsHandler.readLocal(parameter);
       }
 
       Element target = constructor.definingConstructor.implementation;
-      Selector.addForwardingElementArgumentsToList(
+      bool match = Selector.addForwardingElementArgumentsToList(
           constructor,
           arguments,
           target,
           compileArgument,
           handleConstantForOptionalParameter,
           compiler);
+      if (!match) {
+        // If this fails, the selector we constructed for the call to a
+        // forwarding constructor in a mixin application did not match the
+        // constructor (which, for example, may happen when the libraries are
+        // not compatible for private names, see issue 20394).
+        compiler.internalError(constructor,
+                               'forwarding constructor call does not match');
+      }
       inlineSuperOrRedirect(
           target,
           arguments,
           constructors,
           fieldValues,
           constructor);
       return;
     }
     ast.FunctionExpression functionNode = constructor.node;
 
@@ skipping 4637 matching lines @@
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls)));
   }
 }