Remove support for interface in dart2js.

sdk/lib/_internal/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.
 
 part of dart_backend;
 
 class LocalPlaceholder {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
@@ skipping 554 matching lines @@
         makeOmitDeclarationTypePlaceholder(
             parameter.asVariableDefinitions().type);
       }
     }
   }
 
   visitClassNode(ClassNode node) {
     ClassElement classElement = currentElement;
     makeElementPlaceholder(node.name, classElement);
     node.visitChildren(this);
-    if (node.defaultClause != null) {
-      // Can't just visit class node's default clause because of the bug in the
-      // resolver, it just crashes when it meets type variable.
-      DartType defaultType = classElement.defaultClass;
-      assert(defaultType != null);
-      makeTypePlaceholder(node.defaultClause.typeName, defaultType);
-      visit(node.defaultClause.typeArguments);
-    }
   }
 
   visitNamedMixinApplication(NamedMixinApplication node) {
     ClassElement classElement = currentElement;
     makeElementPlaceholder(node.name, classElement);
     node.visitChildren(this);
   }
 
   bool tryResolveAndCollectTypeVariable(
       TypeDeclarationElement typeDeclaration, Identifier name) {
-    // 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 (typeDeclaration is ClassElement
-        && (typeDeclaration as ClassElement).defaultClass != null) {
-      typeDeclaration = (typeDeclaration as ClassElement).defaultClass.element;
-    }
     // Another poor man type resolution.
     // Find this variable in enclosing type declaration parameters.
     for (DartType type in typeDeclaration.typeVariables) {
       if (type.name.slowToString() == name.source.slowToString()) {
         makeTypePlaceholder(name, type);
         return true;
       }
     }
     return false;
   }
@@ skipping 14 matching lines @@
 
   visitBlock(Block node) {
     for (Node statement in node.statements.nodes) {
       if (statement is VariableDefinitions) {
         makeVarDeclarationTypePlaceholder(statement);
       }
     }
     node.visitChildren(this);
   }
 }