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

pkg/compiler/lib/src/io/line_column_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.line_column;
-
-import 'code_output.dart' show CodeOutputListener;
-
-/// Interface for providing line/column information.
-abstract class LineColumnProvider {
-  /// Returns the line number (0-based) for [offset].
-  int getLine(int offset);
-
-  /// Returns the column number (0-based) for [offset] at the given [line].
-  int getColumn(int line, int offset);
-
-  /// Returns the offset for 0-based [line] and [column] numbers.
-  int getOffset(int line, int column);
-}
-
-/// [CodeOutputListener] that collects line information.
-class LineColumnCollector extends CodeOutputListener
-    implements LineColumnProvider {
-  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
-  int getLine(int offset) {
-    List<int> starts = lineStarts;
-    if (offset < 0 || starts.last <= offset) {
-      throw 'bad position #$offset in buffer with length ${length}.';
-    }
-    int first = 0;
-    int count = starts.length;
-    while (count > 1) {
-      int step = count ~/ 2;
-      int middle = first + step;
-      int lineStart = starts[middle];
-      if (offset < lineStart) {
-        count = step;
-      } else {
-        first = middle;
-        count -= step;
-      }
-    }
-    return first;
-  }
-
-  @override
-  int getColumn(int line, int offset) {
-    return offset - lineStarts[line];
-  }
-
-  int getOffset(int line, int column) => lineStarts[line] + column;
-
-  @override
-  void onDone(int length) {
-    lineStarts.add(length + 1);
-    this.length = length;
-  }
-
-  String toString() {
-    return 'lineStarts=$lineStarts,length=$length';
-  }
-}