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

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

 // Copyright (c) 2015, 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.
 
 library dart2js.source_information;
 
+import 'package:kernel/ast.dart' show Location;
 import '../common.dart';
 import '../elements/elements.dart'
     show
         AstElement,
         CompilationUnitElement,
         LocalElement,
         ResolvedAst,
         ResolvedAstKind;
 import '../js/js.dart' show JavaScriptNodeSourceInformation;
 import '../script.dart';
@@ skipping 178 matching lines @@
 
   String toString() {
     // Use 1-based line/column info to match usual dart tool output.
     return '${sourceUri}:[${line + 1},${column + 1}]';
   }
 }
 
 /// A location in a source file.
 abstract class AbstractSourceLocation extends SourceLocation {
   final SourceFile _sourceFile;
-  int _line;
+  Location _location;
 
   AbstractSourceLocation(this._sourceFile) {
     assert(invariant(new SourceSpan(sourceUri, 0, 0), isValid,
         message: "Invalid source location in ${sourceUri}: "
             "offset=$offset, length=${_sourceFile.length}."));
   }
 
   /// The absolute URI of the source file of this source location.
   Uri get sourceUri => _sourceFile.uri;
 
   /// The character offset of the this source location into the source file.
   int get offset;
 
   /// The 0-based line number of the [offset].
   int get line {
-    if (_line == null) _line = _sourceFile.getLine(offset);
-    return _line;
+    _location ??= _sourceFile.getLocation(offset);
+    return _location.line - 1;
   }
 
   /// The 0-base column number of the [offset] with its line.
-  int get column => _sourceFile.getColumn(line, offset);
+  int get column {
+    _location ??= _sourceFile.getLocation(offset);
+    return _location.column - 1;
+  }
 
   /// The name associated with this source location, if any.
   String get sourceName;
 
   /// `true` if the offset within the length of the source file.
   bool get isValid => offset < _sourceFile.length;
 
   String get shortText {
     // Use 1-based line/column info to match usual dart tool output.
     return '${sourceUri.pathSegments.last}:[${line + 1},${column + 1}]';
@@ skipping 97 matching lines @@
   @override
   int get line => null;
 
   @override
   int get offset => null;
 
   String get shortName => '<no-location>';
 
   String toString() => '<no-location>';
 }