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

pkg/compiler/lib/src/use_unused_api.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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
 import '../compiler.dart' as api;
 import 'colors.dart' as colors;
 import 'compiler.dart' as compiler;
 import 'constants/constant_system.dart' as constants;
 import 'constants/constructors.dart' as constants;
 import 'constants/evaluation.dart' as constants;
 import 'constants/expressions.dart' as constants;
 import 'constants/values.dart' as constants;
 import 'dart2js.dart' as dart2js;
 import 'elements/resolution_types.dart' as dart_types;
 import 'deferred_load.dart' as deferred_load;
 import 'diagnostics/source_span.dart' as diagnostics;
 import 'elements/elements.dart' as elements;
 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;
 import 'js_emitter/full_emitter/emitter.dart' as full;
 import 'js_emitter/js_emitter.dart' as js_emitter;
 import 'js_emitter/program_builder/program_builder.dart' as program_builder;
 import 'parser/partial_elements.dart'
     show PartialClassElement, PartialFunctionElement;
 import 'resolution/operators.dart' as operators;
 import 'resolution/semantic_visitor.dart' as semantic_visitor;
@@ skipping 187 matching lines @@
 useFilenames() {
   filenames.appendSlash(null);
 }
 
 useSsa(ssa.HInstruction instruction) {
   instruction.isConstantNumber();
   new ssa.HAndOrBlockInformation(null, null, null);
   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() {
   // TODO(ahe): We should try to avoid including API used only for tests. In
   // most cases, such API can be moved to a test library.
   ClosedWorldImpl closedWorld = null;
   type_graph_inferrer.TypeGraphInferrer typeGraphInferrer = null;
   source_file_provider.SourceFileProvider sourceFileProvider = null;
   sourceFileProvider.getSourceFile(null);
   closedWorld.hasAnyUserDefinedGetter(null, null);
@@ skipping 41 matching lines @@
   operators.UnaryOperator.fromKind(null);
   operators.BinaryOperator.fromKind(null);
   new semantic_visitor.BulkSendVisitor()..apply(null, null);
   new semantic_visitor.TraversalVisitor(null).apply(null, null);
   new semantic_visitor.BulkDeclarationVisitor().apply(null, null);
 }
 
 useDeferred([deferred_load.DeferredLoadTask task]) {
   task.dump();
 }