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 4037 matching lines @@
   }
 
   transformChildren(Transformer v) {
     transformList(libraries, v, this);
   }
 
   Program get enclosingProgram => this;
 
   /// 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);
   }
 }
 
 /// A tuple with file, line, and column number, for displaying human-readable
 /// locations.
 class Location {
   final String file;
   final int line; // 1-based.
   final int column; // 1-based.
 
@@ skipping 85 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. The returned line contains no line separators.
+  String getTextLine(int line) {
+    _rangeCheck(line, 1, lineStarts.length, "line");
+    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 endOfLine = lineStarts[index + 1] - 1;
+      if (endOfLine > index && cachedText[endOfLine - 1] == "\r") {
+        --endOfLine; // Windows line endings.
+      }
+      return cachedText.substring(lineStarts[index], endOfLine);
+    }
+    // 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) {
+    _rangeCheck(offset, 0, lineStarts.last, "offset");
+    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);
+  }
+}
+
+void _rangeCheck(int value, int min, int max, String name) {
+  RangeError.checkValueInInterval(value, min, max, name,
+      "The value of '$name' ($value) must be between $min and $max.");

[sra1, 2017/05/04 05:44:39]

Please remove this interpolation.

It is expensive to interpolate this string for every range check.
I measure ~1% of dart2js time in allocating the strings for _Smi.toString
The information in this interpolated string is pretty redundant with what the
RangeError will print without a 'message' parameter.

If you remove this parameter you will get an error like:

RangeError (offset): Invalid value: Not in range 0..20, inclusive: 100

So you could delete this helper and call RangeError.checkValueInRange from the
two call sites.

 }
 
 /// 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;
 }