Implement reflecting on mixin applications.

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 get currentElement;
   Set<Node> get superUses;
 
@@ skipping 3329 matching lines @@
       link = link.tail;
     }
     doApplyMixinTo(element, supertype, resolveType(link.head));
     return element.computeType(compiler);
   }
 
   DartType applyMixin(DartType supertype, DartType mixinType, Node node) {
     String superName = supertype.name.slowToString();
     String mixinName = mixinType.name.slowToString();
     ClassElement mixinApplication = new MixinApplicationElementX(
-        new SourceString("${superName}_${mixinName}"),
+        new SourceString("${superName}+${mixinName}"),
         element.getCompilationUnit(),
         compiler.getNextFreeClassId(),
         node,
         Modifiers.EMPTY);  // TODO(kasperl): Should this be abstract?
     doApplyMixinTo(mixinApplication, supertype, mixinType);
     mixinApplication.resolutionState = STATE_DONE;
     mixinApplication.supertypeLoadState = STATE_DONE;
     return mixinApplication.computeType(compiler);
   }
 
@@ skipping 725 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }