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..5cb1983464d25255fe038e6b64eca11bcb4aa9ea 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';
 
@@ -4107,22 +4109,7 @@ class Program extends TreeNode {
 
   /// Translates an offset to line and column numbers in the given file.
   Location getLocation(String file, int offset) {
-    List<int> lines = uriToSource[file].lineStarts;
-    int low = 0, high = lines.length - 1;
-    while (low < high) {
-      int mid = high - ((high - low) >> 1); // Get middle, rounding up.
-      int pivot = lines[mid];
-      if (pivot <= offset) {
-        low = mid;
-      } else {
-        high = mid - 1;
-      }
-    }
-    int lineIndex = low;
-    int lineStart = lines[lineIndex];
-    int lineNumber = 1 + lineIndex;
-    int columnNumber = 1 + offset - lineStart;
-    return new Location(file, lineNumber, columnNumber);
+    return uriToSource[file]?.getLocation(file, offset);
   }
 }
 
@@ -4228,9 +4215,73 @@ 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. The returned line contains no line separators.
+  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;
+
+    cachedText ??= UTF8.decode(source, allowMalformed: true);
+    // -1 as line numbers start at 1.
+    int index = line - 1;
+    if (index + 1 == lineStarts.length) {
+      // Last line.
+      return cachedText.substring(lineStarts[index]);
+    } else if (index < lineStarts.length) {
+      // We subtract 1 from the next line for two reasons:
+      // 1. If the file isn't terminated by a newline, that index is invalid.
+      // 2. To remove the newline at the end of the line.
+      int nextLine = lineStarts[index + 1] - 1;
+      String result = cachedText.substring(lineStarts[index], nextLine);
+      if (result.endsWith("\r")) {
+        // Support for Windows text files.
+        result = result.substring(0, result.length - 1);

[asgerf, 2017/04/03 09:55:25]

The 'nextLine' variable does not actually refer to the start of the next line.
'endOfLine' would be a better name.

I also think it's bad practice to create a substring of a substring and then
throw out the first one.

Could we compute the offset directly instead? E.g.

int startOfNextLine = lineStarts[index + 1];
int endOfLine = startOfNextLine - 1; // Remove linebreak.
if (endOfLine > index && 
    cachedText[endOfLine - 1] == '\r') { 
  --endOfLine;  // Handle two-byte linebreak.
}

[ahe, 2017/04/03 14:57:08]

Done.

+      }
+      return result;
+    }
+    // This shouldn't happen: should have been caught by the range check above.
+    throw "Internal error";
+  }
+
+  /// Translates an offset to line and column numbers in the given file.
+  Location getLocation(String file, int offset) {
+    RangeError.checkValueInInterval(
+        offset,
+        0,
+        lineStarts.last,
+        "offset",
+        "The value of 'offset' ($offset) must be between 1 and "

[asgerf, 2017/04/03 09:55:25]

between 1 and -> between 0 and

[ahe, 2017/04/03 14:57:08]

Done.

+        "${lineStarts.length}.");
+
+    int low = 0, high = lineStarts.length - 1;
+    while (low < high) {
+      int mid = high - ((high - low) >> 1); // Get middle, rounding up.
+      int pivot = lineStarts[mid];
+      if (pivot <= offset) {
+        low = mid;
+      } else {
+        high = mid - 1;
+      }
+    }
+    int lineIndex = low;
+    int lineStart = lineStarts[lineIndex];
+    int lineNumber = 1 + lineIndex;
+    int columnNumber = 1 + offset - lineStart;
+    return new Location(file, lineNumber, columnNumber);
+  }
 }
 
 /// Returns the [Reference] object for the given member.