Add Source.getTextLine and use it to display source snippets in error messages.

pkg/compiler/lib/src/io/source_file.dart

 // Copyright (c) 2012, 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 dart2js.io.source_file;
 
 import 'dart:convert' show UTF8;
 import 'dart:math';
 import 'dart:typed_data' show Uint8List;
 
@@ skipping 175 matching lines @@
       }
     }
 
     return buf.toString();
   }
 
   int get lines => lineStarts.length - 1;
 
   /// Returns the text of line  at the 0-based [index] within this source file.
   String getLineText(int index) {
+    // TODO(ahe): This functionality can be replaced by `Source.getTextLine` in
+    // [package:kernel/ast.dart](../../../../kernel/lib/ast.dart).

[asgerf, 2017/04/03 08:34:55]

Any reason not to resolve this now?

[ahe, 2017/04/03 09:26:39]

I hadn't realized how easy that was before you asked :-)

+
     // +1 for 0-indexing, +1 again to avoid the last line of the file
     if ((index + 2) < lineStarts.length) {
       return slowSubstring(lineStarts[index], lineStarts[index + 1]);
     } else if ((index + 1) < lineStarts.length) {
       return '${slowSubstring(lineStarts[index], length)}\n';
     } else {
       throw new ArgumentError("Line index $index is out of bounds.");
     }
   }
 }
@@ skipping 79 matching lines @@
   set length(int v) {}
 
   String slowText() => text;
 
   List<int> slowUtf8ZeroTerminatedBytes() {
     return _zeroTerminateIfNecessary(UTF8.encode(text));
   }
 
   String slowSubstring(int start, int end) => text.substring(start, end);
 }