Move all flags to CompilerOptions (first step to stop passing the compiler to

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 894bd92e56efc401030198ecea22fc40b151276c..09eabafff805dd6c5897f6d512d6c92a4a5f95b3 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
@@ -99,7 +99,7 @@ class FieldVisitor {
         js.Name accessorName = namer.fieldAccessorName(field);
         js.Name fieldName = namer.fieldPropertyName(field);
         bool needsCheckedSetter = false;
-        if (compiler.enableTypeAssertions
+        if (compiler.options.enableTypeAssertions
             && needsSetter
             && !canAvoidGeneratedCheckedSetter(field)) {
           needsCheckedSetter = true;