Alternative fix for the js-interop crash.

pkg/compiler/lib/src/js_backend/js_interop_analysis.dart

 // Copyright (c) 2015, 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.
 
 /// Analysis to determine how to generate code for typed JavaScript interop.
 library compiler.src.js_backend.js_interop_analysis;
 
 import '../common/names.dart' show Identifiers;
 import '../compiler.dart' show Compiler;
 import '../diagnostics/messages.dart' show MessageKind;
@@ skipping 102 matching lines @@
       processJsInteropAnnotation(element);
       if (!backend.isJsInterop(element)) return;
       if (element is FunctionElement) {
         _checkFunctionParameters(element);
       }
 
       if (!element.isClass) return;
 
       ClassElement classElement = element;
 
+      // Skip classes that are completely unreachable. This should only happen
+      // when all of jsinterop types are unreachable from main.
+      if (!backend.compiler.world.isImplemented(classElement)) return;
+
       if (!classElement
           .implementsInterface(helpers.jsJavaScriptObjectClass)) {
         backend.reporter.reportErrorMessage(classElement,
             MessageKind.JS_INTEROP_CLASS_CANNOT_EXTEND_DART_CLASS, {
           'cls': classElement.name,
           'superclass': classElement.superclass.name
         });
       }
 
       classElement.forEachMember(
@@ skipping 51 matching lines @@
           var name = backend.namer.invocationName(selector);
           statements.add(js.statement(
               'Function.prototype.# = function(#) { return this(#) }',
               [name, parameters, parameters]));
         }
       });
     });
     return new jsAst.Block(statements);
   }
 }