Implement override-based inference of instance fields.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index f399fa34020c504e66294ce0281f024fddba4bef..94839faa7f30f88ee44dbe0e796bfa8ed5efc5bb 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -690,6 +690,23 @@ class KernelField extends Field {
   KernelTypeInferrer _typeInferrer;
 
   KernelField(Name name, {String fileUri}) : super(name, fileUri: fileUri) {}
+
+  static FieldNode getFieldNode(Field field) {
+    if (field is KernelField) return field._fieldNode;
+    return null;
+  }
+
+  static void recordOverride(
+      KernelField field, Member overriddenMember, bool isSetter) {
+    if (field._fieldNode != null) {
+      if (isSetter && overriddenMember is Field) {
+        // When overriding a field, we are called twice; once for the setter and
+        // once for the getter.  Ignore the setter.
+        return;
+      }
+      field._fieldNode.overrides.add(overriddenMember);
+    }
+  }
 }
 
 /// Concrete shadow object representing a for-in loop in kernel form.