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

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

Index: pkg/compiler/lib/src/io/source_map_builder.dart
diff --git a/pkg/compiler/lib/src/io/source_map_builder.dart b/pkg/compiler/lib/src/io/source_map_builder.dart
index d1b63dadc349a0dbbcec1dc1d22fd435ba4a2c03..de2abfb052e46820342f2551b3788d8b2f0ce9ac 100644
--- a/pkg/compiler/lib/src/io/source_map_builder.dart
+++ b/pkg/compiler/lib/src/io/source_map_builder.dart
@@ -4,10 +4,11 @@
 
 library dart2js.source_map_builder;
 
+import 'package:kernel/ast.dart' show Location;
 import '../../compiler_new.dart' show OutputSink, OutputType;
 import '../util/uri_extras.dart' show relativize;
 import '../util/util.dart';
-import 'line_column_provider.dart';
+import 'location_provider.dart';
 import 'code_output.dart' show SourceLocationsProvider, SourceLocations;
 import 'source_information.dart' show SourceLocation;
 
@@ -20,11 +21,11 @@ class SourceMapBuilder {
   /// The URI of the target language file.
   final Uri targetFileUri;
 
-  final LineColumnProvider lineColumnProvider;
+  final LocationProvider locationProvider;
   final List<SourceMapEntry> entries = new List<SourceMapEntry>();
 
   SourceMapBuilder(this.version, this.sourceMapUri, this.targetFileUri,
-      this.lineColumnProvider);
+      this.locationProvider);
 
   void addMapping(int targetOffset, SourceLocation sourceLocation) {
     entries.add(new SourceMapEntry(sourceLocation, targetOffset));
@@ -49,9 +50,10 @@ class SourceMapBuilder {
     Map<Uri, LineColumnMap<SourceMapEntry>> sourceLocationMap =
         <Uri, LineColumnMap<SourceMapEntry>>{};
     entries.forEach((SourceMapEntry sourceMapEntry) {
-      int line = lineColumnProvider.getLine(sourceMapEntry.targetOffset);
-      int column =
-          lineColumnProvider.getColumn(line, sourceMapEntry.targetOffset);
+      Location kernelLocation =
+          locationProvider.getLocation(sourceMapEntry.targetOffset);
+      int line = kernelLocation.line - 1;
+      int column = kernelLocation.column - 1;
       lineColumnMap.add(line, column, sourceMapEntry);
 
       SourceLocation location = sourceMapEntry.sourceLocation;
@@ -182,13 +184,13 @@ class SourceMapBuilder {
   }
 
   /// Generates source map files for all [SourceLocations] in
-  /// [sourceLocationsProvider] for the .js code in [lineColumnProvider]
+  /// [sourceLocationsProvider] for the .js code in [locationProvider]
   /// [sourceMapUri] is used to relativizes the URIs of the referenced source
   /// files and the target [fileUri]. [name] and [outputProvider] are used to
   /// create the [OutputSink] for the source map text.
   static void outputSourceMap(
       SourceLocationsProvider sourceLocationsProvider,
-      LineColumnProvider lineColumnProvider,
+      LocationProvider locationProvider,
       String name,
       Uri sourceMapUri,
       Uri fileUri,
@@ -200,7 +202,7 @@ class SourceMapBuilder {
     sourceLocationsProvider.sourceLocations
         .forEach((SourceLocations sourceLocations) {
       SourceMapBuilder sourceMapBuilder = new SourceMapBuilder(
-          sourceLocations.name, sourceMapUri, fileUri, lineColumnProvider);
+          sourceLocations.name, sourceMapUri, fileUri, locationProvider);
       sourceLocations.forEachSourceLocation(sourceMapBuilder.addMapping);
       String sourceMap = sourceMapBuilder.build();
       String extension = 'js.map';