Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart

Index: pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart b/pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart
index 2e19299a5fdc8756767dbf95000cc6ae22e76b09..7f1ac60b19c23100be2d9fbee006d608cb6ce678 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart
@@ -140,7 +140,7 @@ class FieldVisitor {
   bool fieldNeedsGetter(VariableElement field) {
     assert(field.isField);
     if (fieldAccessNeverThrows(field)) return false;
-    if (backend.shouldRetainGetter(field)) return true;
+    if (backend.mirrorsData.shouldRetainGetter(field)) return true;
     return field.isClassMember &&
         compiler.codegenWorldBuilder.hasInvokedGetter(field, closedWorld);
   }
@@ -149,7 +149,7 @@ class FieldVisitor {
     assert(field.isField);
     if (fieldAccessNeverThrows(field)) return false;
     if (field.isFinal || field.isConst) return false;
-    if (backend.shouldRetainSetter(field)) return true;
+    if (backend.mirrorsData.shouldRetainSetter(field)) return true;
     return field.isClassMember &&
         compiler.codegenWorldBuilder.hasInvokedSetter(field, closedWorld);
   }