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

pkg/compiler/lib/src/io/source_file.dart

Index: pkg/compiler/lib/src/io/source_file.dart
diff --git a/pkg/compiler/lib/src/io/source_file.dart b/pkg/compiler/lib/src/io/source_file.dart
index 8b137c0610919f8c4345b7cec46939ed371d9c5c..1cf073c5aaf27ca1d4264e3c3d3a1acdef151105 100644
--- a/pkg/compiler/lib/src/io/source_file.dart
+++ b/pkg/compiler/lib/src/io/source_file.dart
@@ -8,6 +8,8 @@ import 'dart:convert' show UTF8;
 import 'dart:math';
 import 'dart:typed_data' show Uint8List;
 
+import 'package:kernel/ast.dart' as kernel show Location, Source;
+
 import 'line_column_provider.dart';
 
 /**
@@ -18,6 +20,13 @@ abstract class SourceFile implements LineColumnProvider {
   /// The absolute URI of the source file.
   Uri get uri;
 
+  kernel.Source cachedKernelSource;
+
+  kernel.Source get kernelSource {
+    return cachedKernelSource ??= new kernel.Source(
+        lineStarts, slowUtf8ZeroTerminatedBytes())..cachedText = slowText();
+  }
+
   /// The name of the file.
   ///
   /// This is [uri], maybe relativized to a more human-readable form.
@@ -86,25 +95,7 @@ abstract class SourceFile implements LineColumnProvider {
    * representation of this source file.
    */
   int getLine(int position) {
-    List<int> starts = lineStarts;
-    if (position < 0 || starts.last <= position) {
-      throw 'bad position #$position in file $filename 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 (position < lineStart) {
-        count = step;
-      } else {
-        first = middle;
-        count -= step;
-      }
-    }
-    return first;
+    new kernel.Source(lineStarts, null).getLocation(null, position).line;
   }
 
   /**
@@ -141,10 +132,12 @@ abstract class SourceFile implements LineColumnProvider {
       end = length;
     }
 
-    int lineStart = getLine(start);
-    int columnStart = getColumn(lineStart, start);
-    int lineEnd = getLine(end);
-    int columnEnd = getColumn(lineEnd, end);
+    kernel.Location startLocation = kernelSource.getLocation(null, start);
+    kernel.Location endLocation = kernelSource.getLocation(null, end);
+    int lineStart = startLocation.line - 1;
+    int columnStart = startLocation.column - 1;
+    int lineEnd = startLocation.line - 1;
+    int columnEnd = endLocation.column - 1;
 
     StringBuffer buf = new StringBuffer('${filename}:');
     if (start != end || start != 0) {
@@ -155,12 +148,12 @@ abstract class SourceFile implements LineColumnProvider {
 
     if (start != end && includeSourceLine) {
       if (lineStart == lineEnd) {
-        String textLine = getLineText(lineStart);
+        String textLine = kernelSource.getTextLine(startLocation.line);
 
         int toColumn = min(columnStart + (end - start), textLine.length);
         buf.write(textLine.substring(0, columnStart));
         buf.write(colorize(textLine.substring(columnStart, toColumn)));
-        buf.write(textLine.substring(toColumn));
+        buf.writeln(textLine.substring(toColumn));
 
         int i = 0;
         for (; i < columnStart; i++) {
@@ -172,15 +165,15 @@ abstract class SourceFile implements LineColumnProvider {
         }
       } else {
         for (int line = lineStart; line <= lineEnd; line++) {
-          String textLine = getLineText(line);
+          String textLine = kernelSource.getTextLine(line + 1);
           if (line == lineStart) {
             buf.write(textLine.substring(0, columnStart));
-            buf.write(colorize(textLine.substring(columnStart)));
+            buf.writeln(colorize(textLine.substring(columnStart)));
           } else if (line == lineEnd) {
             buf.write(colorize(textLine.substring(0, columnEnd)));
-            buf.write(textLine.substring(columnEnd));
+            buf.writeln(textLine.substring(columnEnd));
           } else {
-            buf.write(colorize(textLine));
+            buf.writeln(colorize(textLine));
           }
         }
       }
@@ -191,17 +184,9 @@ abstract class SourceFile implements LineColumnProvider {
 
   int get lines => lineStarts.length - 1;
 
-  /// Returns the text of line  at the 0-based [index] within this source file.
-  String getLineText(int index) {
-    // +1 for 0-indexing, +1 again to avoid the last line of the file
-    if ((index + 2) < lineStarts.length) {
-      return slowSubstring(lineStarts[index], lineStarts[index + 1]);
-    } else if ((index + 1) < lineStarts.length) {
-      return '${slowSubstring(lineStarts[index], length)}\n';
-    } else {
-      throw new ArgumentError("Line index $index is out of bounds.");
-    }
-  }
+  /// Returns the text of line at the 0-based [index] within this source
+  /// file. The returned line always ends with `"\n"`.
+  String getLineText(int index) => kernelSource.getTextLine(line) + "\n";
 }
 
 List<int> _zeroTerminateIfNecessary(List<int> bytes) {