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

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

Index: pkg/compiler/lib/src/io/source_file.dart
diff --git a/pkg/compiler/lib/src/io/source_file.dart b/pkg/compiler/lib/src/io/source_file.dart
index 8b137c0610919f8c4345b7cec46939ed371d9c5c..cce45563dd8d1eca33dde939e04d5af951f3f4c2 100644
--- a/pkg/compiler/lib/src/io/source_file.dart
+++ b/pkg/compiler/lib/src/io/source_file.dart
@@ -193,6 +193,9 @@ abstract class SourceFile implements LineColumnProvider {
 
   /// 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]);