Fix bug in how AsFilteredImmutableMap was used.

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
index 825498e0ad61881c81aa11cb14dd42338f3fc214..9888b0468a7b6e0ebb530dd50ac16b3d4327312f 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
@@ -518,7 +518,7 @@ abstract class Dart2JsContainerMirror extends Dart2JsElementMirror
     _ensureMembers();
     return new AsFilteredImmutableMap<String, MemberMirror, MethodMirror>(
         _members,
-        (MemberMirror member) => member is MethodMirror);
+        (MemberMirror member) => member is MethodMirror ? member : null);
   }
 
   Map<String, MethodMirror> get getters {
@@ -526,7 +526,8 @@ abstract class Dart2JsContainerMirror extends Dart2JsElementMirror
     return new AsFilteredImmutableMap<String, MemberMirror, MethodMirror>(
         _members,
         (MemberMirror member) =>
-            member is MethodMirror && (member as MethodMirror).isGetter);
+            member is MethodMirror && (member as MethodMirror).isGetter ?
+                member : null);
   }
 
   Map<String, MethodMirror> get setters {
@@ -534,14 +535,15 @@ abstract class Dart2JsContainerMirror extends Dart2JsElementMirror
     return new AsFilteredImmutableMap<String, MemberMirror, MethodMirror>(
         _members,
         (MemberMirror member) =>
-            member is MethodMirror && (member as MethodMirror).isSetter);
+            member is MethodMirror && (member as MethodMirror).isSetter ?
+                member : null);
   }
 
   Map<String, VariableMirror> get variables {
     _ensureMembers();
     return new AsFilteredImmutableMap<String, MemberMirror, VariableMirror>(
         _members,
-        (MemberMirror member) => member is VariableMirror);
+        (MemberMirror member) => member is VariableMirror ? member : null);
   }
 }