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

tests/compiler/dart2js/sourcemaps/diff.dart

Index: tests/compiler/dart2js/sourcemaps/diff.dart
diff --git a/tests/compiler/dart2js/sourcemaps/diff.dart b/tests/compiler/dart2js/sourcemaps/diff.dart
index 7505892879b7f86bd67e3a519048c82504685e47..c3d02b078df2b7c035d9107bcb7cafc7433226c5 100644
--- a/tests/compiler/dart2js/sourcemaps/diff.dart
+++ b/tests/compiler/dart2js/sourcemaps/diff.dart
@@ -540,8 +540,8 @@ class DiffCreator {
     line.htmlParts.add(new ConstHtmlPart('</span>'));
     lines.add(line);
     if (codeSource.begin != null) {
-      int startLine = sourceFile.getLine(codeSource.begin);
-      int endLine = sourceFile.getLine(codeSource.end) + 1;
+      int startLine = sourceFile.getLocation(codeSource.begin).line - 1;
+      int endLine = sourceFile.getLocation(codeSource.end).line;
       for (CodeLine codeLine in convertAnnotatedCodeToCodeLines(
           sourceFile.slowText(), const <Annotation>[],
           startLine: startLine, endLine: endLine)) {
@@ -592,8 +592,8 @@ class DiffCreator {
             currentCodeSource = null;
             for (CodeSource codeSource in codeSources) {
               Interval interval = new Interval(
-                  sourceFile.getLine(codeSource.begin),
-                  sourceFile.getLine(codeSource.end) + 1);
+                  sourceFile.getLocation(codeSource.begin).line - 1,
+                  sourceFile.getLocation(codeSource.end).line);
               if (interval.contains(dartCodeLine.lineNo)) {
                 currentCodeSource = codeSource;
                 currentLineInterval = interval;
@@ -639,7 +639,7 @@ class DiffCreator {
 
         for (CodeLocation location in codeLineAnnotation.codeLocations) {
           SourceFile sourceFile = sourceFileManager.getSourceFile(location.uri);
-          int line = sourceFile.getLine(location.offset);
+          int line = sourceFile.getLocation(location.offset).line - 1;
           if (currentUri != location.uri) {
             restart(jsCodeLine, location, line);
           } else if (interval.inWindow(line, windowSize: 2)) {