Flush cached data in JavaScriptConstantCompiler.metadataConstantMap.

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

Index: dart/sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
index 163b0a14229d0a064ea37b7dbc32d3deedbec4ac..d7541589f2a3686a4663b4b6371b31b690664034 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -493,6 +493,8 @@ abstract class Backend {
   FunctionElement helperForMissingMain() => null;
 
   FunctionElement helperForMainArity() => null;
+
+  void forgetElement(Element element) {}
 }
 
 /// Backend callbacks function specific to the resolution phase.
@@ -1997,6 +1999,7 @@ abstract class Compiler implements DiagnosticListener {
         closureToClassMapper.forgetElement(closure);
       }
     }
+    backend.forgetElement(element);
   }
 }