Re-apply "Implement operator== and hashCode for bound closures."

sdk/lib/_internal/compiler/implementation/ssa/builder.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/builder.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(revision 23383)
+++ sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(working copy)
@@ -67,8 +67,7 @@
             index++;
           });
         } else {
-          // TODO(ahe): I have disabled type optimizations for
-          // optional arguments as the types are stored in the wrong
+          // BUG(10938): the types are stored in the wrong order.
           // order.
           HTypeList parameterTypes =
               backend.optimisticParameterTypes(element.declaration,