Introduce span with line context

lib/src/span_with_context.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.
+
+library source_span.span;
+
+import 'location.dart';
+import 'span.dart';
+
+/// A class that describes a segment of source text with additional context.
+class SourceSpanWithContext extends SourceSpanBase {
+  /// Text around the span, which includes the line containing this span.
+  final String context;
+
+  /// Creates a new span from [start] to [end] (exclusive) containing [text], in
+  /// the given [context].
+  ///
+  /// [start] and [end] must have the same source URL and [start] must come
+  /// before [end]. [text] must have a number of characters equal to the
+  /// distance between [start] and [end]. [context] must contain [text], and
+  /// [text] should start at `start.column` from the beginning of a line in
+  /// [context].
+  SourceSpanWithContext(
+      SourceLocation start, SourceLocation end, String text, this.context)
+      : super(start, end, text) {
+    var index = context.indexOf(text);
+    if (index == -1) {
+      throw new ArgumentError(
+          'The context line "$context" must contain "$text".');
+    }
+
+    var beginningOfLine = context.lastIndexOf('\n', index) + 1;
+    if (start.column != index - beginningOfLine) {
+      throw new ArgumentError('The span text "$text" must start at '
+          'column ${start.column + 1} in a line within "$context".');
+    }
+  }
+}