Extract out a source_span package from source_maps.

pkg/source_span/test/file_message_test.dart

+// Copyright (c) 2014, 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 'package:source_span/source_span.dart';
+import 'package:source_span/src/colors.dart' as colors;
+
+main() {
+  var file;
+  setUp(() {
+    file = new SourceFile("""
+foo bar baz
+whiz bang boom
+zip zap zop
+""", url: "foo.dart");
+  });
+
+  test("points to the span in the source", () {
+    expect(file.span(4, 7).message("oh no"), equals("""
+line 1, column 5 of foo.dart: oh no
+foo bar baz
+    ^^^"""));
+  });
+
+  test("gracefully handles a missing source URL", () {
+    var span = new SourceFile("foo bar baz").span(4, 7);
+    expect(span.message("oh no"), equals("""
+line 1, column 5: oh no
+foo bar baz
+    ^^^"""));
+  });
+
+  test("highlights the first line of a multiline span", () {
+    expect(file.span(4, 20).message("oh no"), equals("""
+line 1, column 5 of foo.dart: oh no
+foo bar baz
+    ^^^^^^^^"""));
+  });
+
+  test("works for a point span", () {
+    expect(file.location(4).pointSpan().message("oh no"), equals("""
+line 1, column 5 of foo.dart: oh no
+foo bar baz
+    ^"""));
+  });
+
+  test("works for a point span at the end of a line", () {
+    expect(file.location(11).pointSpan().message("oh no"), equals("""
+line 1, column 12 of foo.dart: oh no
+foo bar baz
+           ^"""));
+  });
+
+  test("works for a point span at the end of the file", () {
+    expect(file.location(38).pointSpan().message("oh no"), equals("""
+line 3, column 12 of foo.dart: oh no
+zip zap zop
+           ^"""));
+  });
+
+  test("works for a point span in an empty file", () {
+    expect(new SourceFile("").location(0).pointSpan().message("oh no"),
+        equals("""
+line 1, column 1: oh no
+
+^"""));
+  });
+
+  test("works for a single-line file without a newline", () {
+    expect(new SourceFile("foo bar").span(0, 7).message("oh no"),
+        equals("""
+line 1, column 1: oh no
+foo bar
+^^^^^^^"""));
+  });
+
+  group("colors", () {
+    test("doesn't colorize if color is false", () {
+      expect(file.span(4, 7).message("oh no", color: false), equals("""
+line 1, column 5 of foo.dart: oh no
+foo bar baz
+    ^^^"""));
+    });
+
+    test("colorizes if color is true", () {
+      expect(file.span(4, 7).message("oh no", color: true), equals("""
+line 1, column 5 of foo.dart: oh no
+foo ${colors.RED}bar${colors.NONE} baz
+    ${colors.RED}^^^${colors.NONE}"""));
+    });
+
+    test("uses the given color if it's passed", () {
+      expect(file.span(4, 7).message("oh no", color: colors.YELLOW), equals("""
+line 1, column 5 of foo.dart: oh no
+foo ${colors.YELLOW}bar${colors.NONE} baz
+    ${colors.YELLOW}^^^${colors.NONE}"""));
+    });
+  });
+}