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

pkg/compiler/lib/src/use_unused_api.dart

Index: pkg/compiler/lib/src/use_unused_api.dart
diff --git a/pkg/compiler/lib/src/use_unused_api.dart b/pkg/compiler/lib/src/use_unused_api.dart
index 5a547958206692288e060a5ad10caceb558c1014..f6bb6b8ac4cb2eed0721c845d4f4f26d88cd144a 100644
--- a/pkg/compiler/lib/src/use_unused_api.dart
+++ b/pkg/compiler/lib/src/use_unused_api.dart
@@ -25,7 +25,7 @@ import 'elements/modelx.dart' as modelx;
 import 'elements/visitor.dart' as elements_visitor;
 import 'filenames.dart' as filenames;
 import 'inferrer/type_graph_inferrer.dart' as type_graph_inferrer;
-import 'io/line_column_provider.dart' as io;
+import 'io/location_provider.dart' as io;
 import 'io/source_map_builder.dart' as io;
 import 'js/js.dart' as js;
 import 'js_backend/js_backend.dart' as js_backend;
@@ -233,13 +233,12 @@ useSsa(ssa.HInstruction instruction) {
   new ssa.HStatementSequenceInformation(null);
 }
 
-useIo([io.LineColumnMap map, io.LineColumnProvider provider]) {
+useIo([io.LineColumnMap map, io.LocationProvider provider]) {
   map
     ..addFirst(null, null, null)
     ..forEachLine(null)
     ..getFirstElementsInLine(null)
     ..forEachColumn(null, null);
-  provider.getOffset(null, null);
 }
 
 usedByTests() {