Insert implicit downcasts in kernel strong mode.

pkg/kernel/lib/transformations/insert_type_checks.dart

Index: pkg/kernel/lib/transformations/insert_type_checks.dart
diff --git a/pkg/kernel/lib/transformations/insert_type_checks.dart b/pkg/kernel/lib/transformations/insert_type_checks.dart
new file mode 100644
index 0000000000000000000000000000000000000000..01db445a5e6a2056c54326b9046826a6a07fed93
--- /dev/null
+++ b/pkg/kernel/lib/transformations/insert_type_checks.dart
@@ -0,0 +1,58 @@
+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+library kernel.transformations.insert_type_checks;
+
+import '../ast.dart';
+import '../class_hierarchy.dart';
+import '../core_types.dart';
+import '../log.dart';
+import '../type_checker.dart';
+
+/// Inserts implicit downcasts in method bodies to ensure type safety.
+///
+/// This does not deal with covariant override and covariant use of type
+/// parameters.
+///
+/// Ideally this should be done when initially generating kernel IR, but this
+/// is not practical at the moment.
+class InsertTypeChecks {
+  CoreTypes coreTypes;
+  ClassHierarchy hierarchy;
+
+  InsertTypeChecks({this.coreTypes, this.hierarchy});
+
+  void transformProgram(Program program) {
+    coreTypes ??= new CoreTypes(program);
+    hierarchy ??= new ClassHierarchy(program);
+    new CheckInsertingTypeChecker(coreTypes, hierarchy).checkProgram(program);
+  }
+}
+
+class CheckInsertingTypeChecker extends TypeChecker {
+  CheckInsertingTypeChecker(CoreTypes coreTypes, ClassHierarchy hierarchy)
+      : super(coreTypes, hierarchy);
+
+  @override
+  void fail(TreeNode where, String message) {
+    log.severe('${where.location}: $message');
+  }
+
+  @override
+  void checkAssignable(TreeNode where, DartType from, DartType to) {
+    if (!environment.isSubtypeOf(from, to)) {
+      fail(where, '$from cannot be assigned to $to');
+    }
+  }
+
+  @override
+  Expression checkAndDowncastExpression(
+      Expression expression, DartType from, DartType to) {
+    if (!environment.isSubtypeOf(from, to)) {
+      return new AsExpression(expression, to)
+        ..fileOffset = expression.fileOffset;
+    } else {
+      return expression;
+    }
+  }
+}