kernel -> ssa: implement if-statements

tests/compiler/dart2js/kernel/control_flow_test.dart

+// 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.
+
+import 'package:test/test.dart';
+
+import 'helper.dart' show check;
+
+main() {
+  group('compile control flow', () {
+    test('simple if statement', () {
+      String code = '''
+        main() {
+          if (true) {
+            return 1;
+          } else {
+            return 2;
+          }
+        }''';
+      return check(code);
+    });
+    test('simple if statement no else', () {
+      String code = '''
+        main() {
+          if (true) {
+            return 1;
+          }
+        }''';
+      return check(code);
+    });
+    test('simple dead if statement', () {
+      String code = '''
+        main() {
+          if (false) {
+            return 1;
+          }
+        }''';
+      return check(code);
+    });
+  });
+}