[dart2dart] Fix type variables for interfaces and default classes if they are in different libs.

lib/compiler/implementation/dart_backend/placeholder_collector.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.
 
 class LocalPlaceholder implements Hashable {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
   int hashCode() => identifier.hashCode();
   String toString() =>
@@ skipping 370 matching lines @@
       // resolver, it just crashes when it meets type variable.
       Type defaultType = classElement.defaultClass;
       assert(defaultType !== null);
       makeTypePlaceholder(node.defaultClause.typeName, defaultType);
       visit(node.defaultClause.typeArguments);
     }
   }
 
   visitTypeVariable(TypeVariable node) {
     assert(currentElement is TypedefElement || currentElement is ClassElement);
+    // Hack for case when interface and default class are in different
+    // libraries, try to resolve type variable to default class type arg.
+    // Example:
+    // lib1: interface I<K> default C<K> {...}
+    // lib2: class C<K> {...}
+    if (currentElement is ClassElement
+        && (currentElement as ClassElement).defaultClass !== null) {
+      currentElement = (currentElement as ClassElement).defaultClass.element;
+    }
     // Another poor man type resolution.
     // Find this variable in current element type parameters.
     for (Type type in currentElement.typeVariables) {
       if (type.name.slowToString() == node.name.source.slowToString()) {
         makeTypePlaceholder(node.name, type);
         break;
       }
     }
     node.visitChildren(this);
   }
 
   visitTypedef(Typedef node) {
     assert(currentElement is TypedefElement);
     makeElementPlaceholder(node.name, currentElement);
     node.visitChildren(this);
   }
 }