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

pkg/compiler/lib/src/io/location_provider.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.
+
+library dart2js.io.location_provider;
+
+import 'code_output.dart' show CodeOutputListener;
+
+import 'package:kernel/ast.dart' show Location, Source;
+
+/// Interface for providing line/column information.
+abstract class LocationProvider {
+  /// Translates the zero-based character [offset] (from the beginning of a
+  /// file) to a [Location].
+  Location getLocation(int offset);
+}
+
+/// [CodeOutputListener] that collects line information.
+class LocationCollector extends CodeOutputListener implements LocationProvider {
+  int length = 0;
+  List<int> lineStarts = <int>[0];
+
+  void _collect(String text) {
+    int index = 0;
+    while (index < text.length) {
+      // Unix uses '\n' and Windows uses '\r\n', so this algorithm works for
+      // both platforms.
+      index = text.indexOf('\n', index) + 1;
+      if (index <= 0) break;
+      lineStarts.add(length + index);
+    }
+    length += text.length;
+  }
+
+  @override
+  void onText(String text) {
+    _collect(text);
+  }
+
+  @override
+  Location getLocation(int offset) {
+    RangeError.checkValueInInterval(offset, 0, length, "offset",
+        "The value of 'offset' ($offset) must be between 0 and $length.");
+    return new Source(lineStarts, null).getLocation(null, offset);
+  }
+
+  @override
+  void onDone(int length) {
+    lineStarts.add(length + 1);
+    this.length = length;
+  }
+
+  String toString() {
+    return 'lineStarts=$lineStarts,length=$length';
+  }
+}