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

pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart b/pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart
index 51df6b1fa98af15070ffb7bb4f436e824392e2b2..76536293955f72cb67c0b7d42d2eea274e616871 100644
--- a/pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart
@@ -38,7 +38,7 @@ import '../../common_elements.dart' show CommonElements;
 import '../../elements/elements.dart' show ClassElement, MethodElement;
 import '../../hash/sha1.dart' show Hasher;
 import '../../io/code_output.dart';
-import '../../io/line_column_provider.dart' show LineColumnCollector;
+import '../../io/location_provider.dart' show LocationCollector;
 import '../../io/source_map_builder.dart' show SourceMapBuilder;
 import '../../js/js.dart' as js;
 import '../../js_backend/js_backend.dart'
@@ -242,11 +242,11 @@ class ModelEmitter {
   // Updates the shared [outputBuffers] field with the output.
   void writeMainFragment(MainFragment fragment, js.Statement code,
       {bool isSplit}) {
-    LineColumnCollector lineColumnCollector;
+    LocationCollector locationCollector;
     List<CodeOutputListener> codeOutputListeners;
     if (shouldGenerateSourceMap) {
-      lineColumnCollector = new LineColumnCollector();
-      codeOutputListeners = <CodeOutputListener>[lineColumnCollector];
+      locationCollector = new LocationCollector();
+      codeOutputListeners = <CodeOutputListener>[locationCollector];
     }
 
     CodeOutput mainOutput = new StreamCodeOutput(
@@ -273,7 +273,7 @@ class ModelEmitter {
     if (shouldGenerateSourceMap) {
       SourceMapBuilder.outputSourceMap(
           mainOutput,
-          lineColumnCollector,
+          locationCollector,
           '',
           compiler.options.sourceMapUri,
           compiler.options.outputUri,
@@ -291,10 +291,10 @@ class ModelEmitter {
     Hasher hasher = new Hasher();
     outputListeners.add(hasher);
 
-    LineColumnCollector lineColumnCollector;
+    LocationCollector locationCollector;
     if (shouldGenerateSourceMap) {
-      lineColumnCollector = new LineColumnCollector();
-      outputListeners.add(lineColumnCollector);
+      locationCollector = new LocationCollector();
+      outputListeners.add(locationCollector);
     }
 
     String hunkPrefix = fragment.outputFileName;
@@ -357,7 +357,7 @@ class ModelEmitter {
 
       output.add(SourceMapBuilder.generateSourceMapTag(mapUri, partUri));
       output.close();
-      SourceMapBuilder.outputSourceMap(output, lineColumnCollector, partName,
+      SourceMapBuilder.outputSourceMap(output, locationCollector, partName,
           mapUri, partUri, compiler.outputProvider);
     } else {
       output.close();