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

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index 0112b5a7286a9cfd651dfbe5f032e6e43bbf8ae5..9b531d7b8b92a6b947a902ec23dd98f4545a909e 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -50,6 +50,8 @@
 ///
 library kernel.ast;
 
+import 'dart:convert' show UTF8;
+
 import 'visitor.dart';
 export 'visitor.dart';
 
@@ -4228,9 +4230,36 @@ class _ChildReplacer extends Transformer {
 
 class Source {
   final List<int> lineStarts;
+
   final List<int> source;
 
+  String cachedText;
+
   Source(this.lineStarts, this.source);
+
+  /// Return the text corresponding to [line] which is a 1-based line number.
+  String getTextLine(int line) {
+    RangeError.checkValueInInterval(
+        line,
+        1,
+        lineStarts.length,
+        "line",
+        "The value of 'line' ($line) must be between 1 and "
+        "${lineStarts.length}.");
+    if (source == null) return null;
+
+    // -1 as line numbers start at 1.
+    int index = line - 1;
+    if (index + 1 == lineStarts.length) {
+      // Last line.
+      return cachedText.substring(lineStarts[index]);

[asgerf, 2017/04/03 08:34:55]

cachedText could be null here. The initialization below should be moved up.

[ahe, 2017/04/03 09:26:39]

Done.

+    } else if (index < lineStarts.length) {
+      cachedText ??= UTF8.decode(source, allowMalformed: true);
+      return cachedText.substring(lineStarts[index], lineStarts[index + 1]);
+    }
+    // This shouldn't happen: should have been caught by the range check above.
+    throw "Internal error";
+  }
 }
 
 /// Returns the [Reference] object for the given member.