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

tests/compiler/dart2js/sourcemaps/sourcemap_html_helper.dart

Index: tests/compiler/dart2js/sourcemaps/sourcemap_html_helper.dart
diff --git a/tests/compiler/dart2js/sourcemaps/sourcemap_html_helper.dart b/tests/compiler/dart2js/sourcemaps/sourcemap_html_helper.dart
index 423c054085fb7047d07ba6da5fd88379c11ba42a..fae4da9bc20e2d3b31db38fe4a4a4aaa0bffa9f0 100644
--- a/tests/compiler/dart2js/sourcemaps/sourcemap_html_helper.dart
+++ b/tests/compiler/dart2js/sourcemaps/sourcemap_html_helper.dart
@@ -515,7 +515,7 @@ String computeDartHtmlPart(String name, SourceFileManager sourceFileManager,
             line++) {
           if (line >= 0) {
             dartCodeBuffer.write(lineNumber(line, width: lineNoWidth));
-            dartCodeBuffer.write(sourceFile.getLineText(line));
+            dartCodeBuffer.write(sourceFile.kernelSource.getTextLine(line + 1));

[asgerf, 2017/04/03 15:19:14]

Wouldn't it be better to keep the name getLineText and consistently accept
0-based line indices instead of 1-based line numbers?

[ahe, 2017/04/03 15:34:55]

I'm not sure how much of this I can change, and how much is given by the source
map format.

[asgerf, 2017/04/04 08:33:59]

It has nothing to do with the source map format.

I meant the method for doing this was called getLineText before this change. 
Now it is called getTextLine.

Where the method previously took N it now takes N+1.

[ahe, 2017/04/04 09:03:07]

Sorry, I misunderstood your question. We discussed this offline, and I'll try to
summarize it below:

* Most tools display line and column numbers starting at 1. So when I saw that
Location was 1-based, I assumed that represented conscious choice to represent
them 1-based. That made sense to me, as almost all tools I've ever worked with
have 1-based line and column numbers. Apparently, I read too much in to it, but
now that we talked about it, we all seem to like that line and column numbers
are represented as you see them.

* I've tried to remove all location-related API from SourceFile, all that's left
is getLocation and kernelSource. This is why I removed getLine, getColumn, and
getLineText from SourceFile. As I reimplemented getLineText in package:kernel, I
felt the name getTextLine was more natural, hence the rename.

* Johnni told me that source maps are also 1-based, so it would actually make
sense to change the class SourceLocation in dart2js, but I feel I'm getting
side-tracked now, so I'm stopping short of fixing that problem.

           }
         }
         dartCodeBuffer.write(codeBuffer);
@@ -524,7 +524,7 @@ String computeDartHtmlPart(String name, SourceFileManager sourceFileManager,
             line++) {
           if (line < sourceFile.lines) {
             dartCodeBuffer.write(lineNumber(line, width: lineNoWidth));
-            dartCodeBuffer.write(sourceFile.getLineText(line));
+            dartCodeBuffer.write(sourceFile.kernelSource.getTextLine(line + 1));
           }
         }
         dartCodeBuffer.write('</pre>\n');
@@ -544,10 +544,10 @@ String computeDartHtmlPart(String name, SourceFileManager sourceFileManager,
       } else {
         for (int line = lastLineIndex + 1; line < lineIndex; line++) {
           codeBuffer.write(lineNumber(line, width: lineNoWidth));
-          codeBuffer.write(sourceFile.getLineText(line));
+          codeBuffer.write(sourceFile.kernelSource.getTextLine(line + 1));
         }
       }
-      String line = sourceFile.getLineText(lineIndex);
+      String line = sourceFile.kernelSource.getTextLine(lineIndex + 1);
       locations.sort((a, b) => a.offset.compareTo(b.offset));
       for (int i = 0; i < locations.length; i++) {
         SourceLocation sourceLocation = locations[i];