Add some user-friendly utility functions to pkg/analyzer_experimental.

pkg/analyzer_experimental/test/error_test.dart

Index: pkg/analyzer_experimental/test/error_test.dart
diff --git a/pkg/analyzer_experimental/test/error_test.dart b/pkg/analyzer_experimental/test/error_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4a513d95c67e0f92f3f2fa25232db1586c8caf7e
--- /dev/null
+++ b/pkg/analyzer_experimental/test/error_test.dart
@@ -0,0 +1,62 @@
+// Copyright (c) 2013, 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:unittest/unittest.dart';
+
+import 'utils.dart';
+
+void main() {
+  test("a valid Dart file doesn't throw any errors", () {
+    expect(
+        errorsForFile('void main() => print("Hello, world!");'),
+        isNull);
+  });
+
+  test("an error on the first line", () {
+    expect(errorsForFile('void foo;\n'),
+        equals('Error on line 1 of test.dart: ...\n'
+               'void foo;\n'
+               '^^^^\n'));
+  });
+
+  test("an error on the last line", () {
+    expect(errorsForFile('\nvoid foo;'),
+        equals('Error on line 2 of test.dart: ...\n'
+               'void foo;\n'
+               '^^^^\n'));
+  });
+
+  test("an error in the middle", () {
+    expect(errorsForFile('\nvoid foo;\n'),
+        equals('Error on line 2 of test.dart: ...\n'
+               'void foo;\n'
+               '^^^^\n'));
+  });
+
+  var veryLongString = new List.filled(107, ' ').join('');
+
+  test("an error at the end of a very long line", () {
+    expect(errorsForFile('$veryLongString     void foo;'),
+        equals('Error on line 1 of test.dart: ...\n'
+               '...$veryLongString void foo;\n'
+               '$veryLongString    ^^^^\n'));
+  });
+
+  test("an error at the beginning of a very long line", () {
+    expect(errorsForFile('void foo;     $veryLongString'),
+        equals('Error on line 1 of test.dart: ...\n'
+               'void foo; $veryLongString...\n'
+               '^^^^\n'));
+  });
+
+  test("an error in the middle of a very long line", () {
+    expect(errorsForFile('$veryLongString void foo;$veryLongString'),
+        equals('Error on line 1 of test.dart: ...\n'
+               '...                                                         '
+                   'void foo;                                                '
+                   '...\n'
+               '                                                            '
+                   '^^^^\n'));
+  });
+}