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

pkg/kernel/lib/ast.dart

 // Copyright (c) 2016, 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.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 32 matching lines @@
 ///
 /// Use [Transformer] for bulk transformations that are likely to transform lots
 /// of nodes, and [TreeNode.replaceWith] for sparse transformations that mutate
 /// relatively few nodes.  Or use whichever is more convenient.
 ///
 /// The AST can also be mutated by direct field manipulation, but the user then
 /// has to update parent pointers manually.
 ///
 library kernel.ast;
 
+import 'dart:convert' show UTF8;
+
 import 'visitor.dart';
 export 'visitor.dart';
 
 import 'canonical_name.dart' show CanonicalName;
 export 'canonical_name.dart' show CanonicalName;
 
 import 'transformations/flags.dart';
 import 'text/ast_to_text.dart';
 import 'type_algebra.dart';
 import 'type_environment.dart';
@@ skipping 4158 matching lines @@
     if (node == child) {
       return replacement;
     } else {
       return node;
     }
   }
 }
 
 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.
 ///
 /// Returns `null` if the member is `null`.
 Reference getMemberReference(Member member) {
   return member?.reference;
 }
 
 /// Returns the [Reference] object for the given class.
@@ skipping 31 matching lines @@
 /// library has not been assigned a canonical name yet.
 ///
 /// Returns `null` if the library is `null`.
 CanonicalName getCanonicalNameOfLibrary(Library library) {
   if (library == null) return null;
   if (library.canonicalName == null) {
     throw '$library has no canonical name';
   }
   return library.canonicalName;
 }