Introduce span with line context

lib/src/file.dart

Index: lib/src/file.dart
diff --git a/lib/src/file.dart b/lib/src/file.dart
index ed5f6a8371226ed65065789a6ec8ee043701426f..daac9a84969ba594e872befb802066c0605f152a 100644
--- a/lib/src/file.dart
+++ b/lib/src/file.dart
@@ -48,8 +48,7 @@ class SourceFile {
   /// Creates a new source file from [text].
   ///
   /// [url] may be either a [String], a [Uri], or `null`.
-  SourceFile(String text, {url})
-      : this.decoded(text.runes, url: url);
+  SourceFile(String text, {url}) : this.decoded(text.runes, url: url);
 
   /// Creates a new source file from a list of decoded characters.
   ///
@@ -164,8 +163,7 @@ class FileLocation extends SourceLocation {
   int get line => file.getLine(offset);
   int get column => file.getColumn(offset);
 
-  FileLocation._(this.file, int offset)
-      : super(offset) {
+  FileLocation._(this.file, int offset) : super(offset) {
     if (offset > file.length) {
       throw new RangeError("Offset $offset must not be greater than the number "
           "of characters in the file, ${file.length}.");
@@ -183,7 +181,7 @@ class FileLocation extends SourceLocation {
 /// [FileSpan.union] will return a [FileSpan] if possible.
 ///
 /// A [FileSpan] can be created using [SourceFile.span].
-class FileSpan extends SourceSpanMixin {
+class FileSpan extends SourceSpanMixin implements SourceSpanWithContext {
   /// The [file] that [this] belongs to.
   final SourceFile file;
 
@@ -205,6 +203,12 @@ class FileSpan extends SourceSpanMixin {
   FileLocation get end => new FileLocation._(file, _end);
   String get text => file.getText(_start, _end);
 
+  String get context {
+    var line = start.line;
+    return file.getText(file.getOffset(line),
+        line == file.lines - 1 ? null : file.getOffset(line + 1));
+  }
+
   FileSpan._(this.file, this._start, this._end) {
     if (_end < _start) {
       throw new ArgumentError('End $_end must come after start $_start.');
@@ -240,12 +244,13 @@ class FileSpan extends SourceSpanMixin {
 
   bool operator ==(other) {
     if (other is! FileSpan) return super == other;
-    return _start == other._start && _end == other._end &&
+    return _start == other._start &&
+        _end == other._end &&
         sourceUrl == other.sourceUrl;
   }
 
-  int get hashCode => _start.hashCode + 5 * _end.hashCode +
-      7 * sourceUrl.hashCode;
+  int get hashCode =>
+      _start.hashCode + 5 * _end.hashCode + 7 * sourceUrl.hashCode;
 
   /// Returns a new span that covers both [this] and [other].
   ///
@@ -261,41 +266,4 @@ class FileSpan extends SourceSpanMixin {
     var end = math.max(this._end, other._end);
     return new FileSpan._(file, start, end);
   }
-
-  String message(String message, {color}) {
-    if (color == true) color = colors.RED;
-    if (color == false) color = null;
-
-    var line = start.line;
-    var column = start.column;
-
-    var buffer = new StringBuffer();
-    buffer.write('line ${start.line + 1}, column ${start.column + 1}');
-    if (sourceUrl != null) buffer.write(' of ${p.prettyUri(sourceUrl)}');
-    buffer.write(': $message\n');
-
-    var textLine = file.getText(file.getOffset(line),
-        line == file.lines - 1 ? null : file.getOffset(line + 1));
-
-    column = math.min(column, textLine.length - 1);
-    var toColumn =
-        math.min(column + end.offset - start.offset, textLine.length);
-
-    if (color != null) {
-      buffer.write(textLine.substring(0, column));
-      buffer.write(color);
-      buffer.write(textLine.substring(column, toColumn));
-      buffer.write(colors.NONE);
-      buffer.write(textLine.substring(toColumn));
-    } else {
-      buffer.write(textLine);
-    }
-    if (!textLine.endsWith('\n')) buffer.write('\n');
-
-    buffer.write(' ' * column);
-    if (color != null) buffer.write(color);
-    buffer.write('^' * math.max(toColumn - column, 1));
-    if (color != null) buffer.write(colors.NONE);
-    return buffer.toString();
-  }
 }