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

tests/compiler/dart2js/line_column_provider_test.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.
-
-// Unittest for the [LineColumnCollector].
-
-import 'package:expect/expect.dart';
-import 'package:compiler/src/io/code_output.dart';
-import 'package:compiler/src/io/line_column_provider.dart';
-
-import 'output_collector.dart';
-
-test(List events, Map<int, List<int>> expectedPositions) {
-  BufferedOutputSink sink = new BufferedOutputSink();
-  LineColumnProvider lineColumnProvider = new LineColumnCollector();
-  CodeOutput output = new StreamCodeOutput(sink, [lineColumnProvider]);
-  for (var event in events) {
-    if (event is String) {
-      output.add(event);
-    } else if (event is CodeBuffer) {
-      output.addBuffer(event);
-    }
-  }
-  output.close();
-
-  expectedPositions.forEach((int offset, List<int> expectedPosition) {
-    if (expectedPosition == null) {
-      Expect.throws(
-          () => lineColumnProvider.getLine(offset),
-          (e) => true,
-          'Expected out-of-bounds offset: $offset\n'
-          'text:"""${sink.text}"""\n'
-          'lineColumnProvider:$lineColumnProvider');
-    } else {
-      int line = lineColumnProvider.getLine(offset);
-      int column = lineColumnProvider.getColumn(line, offset);
-      Expect.equals(
-          expectedPosition[0],
-          line,
-          'Unexpected result: $offset -> $expectedPosition = [$line,$column]\n'
-          'text:"""${sink.text}"""\n'
-          'lineColumnProvider:$lineColumnProvider');
-      Expect.equals(
-          expectedPosition[1],
-          column,
-          'Unexpected result: $offset -> $expectedPosition = [$line,$column]\n'
-          'text:"""${sink.text}"""\n'
-          'lineColumnProvider:$lineColumnProvider');
-    }
-  });
-}
-
-main() {
-  test([
-    ""
-  ], {
-    0: [0, 0],
-    1: null
-  });
-
-  test([
-    " "
-  ], {
-    0: [0, 0],
-    1: [0, 1],
-    2: null
-  });
-
-  test([
-    "\n "
-  ], {
-    0: [0, 0],
-    1: [1, 0],
-    2: [1, 1],
-    3: null
-  });
-
-  Map positions = {
-    0: [0, 0],
-    1: [0, 1],
-    2: [1, 0],
-    3: [1, 1],
-    4: [2, 0],
-    5: [2, 1],
-    6: null
-  };
-
-  test(["a\nb\nc"], positions);
-
-  test(["a", "\nb\nc"], positions);
-
-  test(["a", "\n", "b\nc"], positions);
-
-  CodeBuffer buffer1 = new CodeBuffer();
-  buffer1.add("a\nb\nc");
-  test([buffer1], positions);
-
-  CodeBuffer buffer2 = new CodeBuffer();
-  buffer2.add("\nb\nc");
-  test(["a", buffer2], positions);
-
-  CodeBuffer buffer3 = new CodeBuffer();
-  buffer3.add("a");
-  test([buffer3, buffer2], positions);
-
-  CodeBuffer buffer4 = new CodeBuffer();
-  buffer4.addBuffer(buffer3);
-  test([buffer4, buffer2], positions);
-}