Fix JS-backend when MirrorsUsed target a static field, but there is no mirrors usage.

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

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 56bf967f898966410c89688f59d86c500995affa..30d0c468f6008440c2c20c51b51a8b5b8748d852 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -1750,7 +1750,28 @@ class JavaScriptBackend extends Backend {
       preMirrorsMethodCount = generatedCode.length;
     }
 
-    if (isTreeShakingDisabled) enqueuer.enqueueEverything();
+    if (isTreeShakingDisabled) {
+      enqueuer.enqueueEverything();
+    } else if (!targetsUsed.isEmpty) {
+      // Add all static elements (not classes) that have been requested for
+      // reflection. If there is no mirror-usage these are probably not
+      // necessary, but the backend relies on them being resolved.
+      List staticFields = [];
+      for (Element target in targetsUsed) {
+        if (target == null) continue;
+        if (target.isField()) {
+          staticFields.add(target);
+        } else if (target.isLibrary() || target.isClass()) {
+          ScopeContainerElement container = target;
+          container.forEachLocalMember((Element member) {
+            if (!member.isInstanceMember() && member.isField()) {
+              staticFields.add(member);
+            }
+          });
+        }
+      }
+      enqueuer.enqueueReflectiveStaticFields(staticFields);
+    }
 
     if (mustPreserveNames) compiler.log('Preserving names.');