Add test of unittests.

tests/compiler/dart2js/parser_test.dart

 // Copyright (c) 2011, 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:expect/expect.dart";
 import 'parser_helper.dart';
 import 'package:compiler/src/tree/tree.dart';
 
 void testStatement(String statement) {
   Node node = parseStatement(statement);
@@ skipping 137 matching lines @@
   node = parseStatement("a ?? b ? c : d;");
   // Should parse as: (a ?? b) ? c : d;
   conditional = node.expression;
   Expect.isNotNull(conditional.condition.asSend());
   Expect.isTrue(conditional.condition.asSend().isIfNull);
   Expect.isNotNull(conditional.thenExpression.asSend());
   Expect.isNotNull(conditional.elseExpression.asSend());
 }
 
 void testNullOperators() {
-  Expression node = parseStatement("a ?? b;").expression;
+  ExpressionStatement statement = parseStatement("a ?? b;");
+  Expression node = statement.expression;
   Expect.isNotNull(node.asSend());
   Expect.isTrue(node.asSend().isIfNull);
 
-  node = parseStatement("a ??= b;").expression;
+  statement = parseStatement("a ??= b;");
+  node = statement.expression;
   Expect.isNotNull(node.asSendSet());
   Expect.isTrue(node.asSendSet().isIfNullAssignment);
 
-  node = parseStatement("a?.b;").expression;
+  statement = parseStatement("a?.b;");
+  node = statement.expression;
   Expect.isNotNull(node.asSend());
   Expect.isTrue(node.asSend().isConditional);
 
-  node = parseStatement("a?.m();").expression;
+  statement = parseStatement("a?.m();");
+  node = statement.expression;
   Expect.isNotNull(node.asSend());
   Expect.isTrue(node.asSend().isConditional);
 }
 
 void testAssignment() {
   ExpressionStatement node;
   Expression expression;
   SendSet sendSet;
 
   node = parseStatement("a = b;");
@@ skipping 190 matching lines @@
   testConditionalExpression();
   testNullOperators();
   testAssignment();
   testIndex();
   testPostfix();
   testOperatorParse();
   testMissingCloseParen();
   testMissingCloseBraceInClass();
   testUnmatchedAngleBracket();
 }